Singleton Preterm Birth Research Articles

Cumulative live birth rate of a blastocyst versus cleavage stage embryo transfer policy during in vitro fertilisation in women with a good prognosis: multicentre randomised controlled trial

ObjectivesTo evaluate whether embryo transfers at blastocyst stage improve the cumulative live birth rate after oocyte retrieval, including both fresh and frozen-thawed transfers, and whether the risk of obstetric and...

Deferred Cord Clamping in Twin Pregnancies Across Canada: A National Survey of Practices

ObjectiveGuidelines recommending deferred cord clamping (DCC), delaying cord clamping for at least 30 seconds post-birth, have shown significant benefits in preterm singleton births. However, evidence supporting DCC in twins is scarce due to limited trial data, leading to practice variations. This study aims to assess current reported DCC practices for twin pregnancies in tertiary hospitals across Canada. MethodsA web-based survey was distributed to neonatologists and obstetrician investigators associated with the Canadian Neonatal and Preterm Birth Networks operating maternity and neonatal units. ResultsThe site response rate was 93% (28/30 sites), with 83% (25/30) for neonatologists and 56% (17/30) for obstetricians. The majority had a local protocol for twin pregnancies (obstetricians 13/17, neonatologists 21/25). While all centres practised DCC in dichorionic-diamniotic twins, a difference was noted for monochorionic-diamniotic twins, with 56% of neonatologists and 65% of obstetricians performing DCC. During cesarean delivery, most obstetricians (76.5%) placed the firstborn on the mother’s thighs. Neonatologists varied in their practices, with 32% placing the baby on the mother’s abdomen, 32% on the mother’s thighs, and 28% holding the baby at the height of the perineum. Divergent opinions were observed regarding contraindications, including risks of postpartum hemorrhage and velamentous cord insertion. ConclusionsDCC is reported to be practised in most twin deliveries among Canadian Neonatal and Preterm Birth Network centres. However, there are wide variations in practice, especially concerning the characteristics of the twins in which DCC is performed. Future research should investigate optimal cord clamping management in twins to standardize practices and maximize benefits.

Ascertainment of Aboriginal and Torres Strait Islander status for assessment of perinatal health outcomes: Reported versus derived maternal ethnicity in Western Australian pregnancy data.

Under-identification of Aboriginal and Torres Strait Islander (hereafter referred to as Aboriginal) people can result in inaccurate estimation of health outcomes. Data linkage has improved identification of Aboriginal people in administrative datasets. To compare three methods of ascertainment of Aboriginal status using only pregnancy data from the Western Australian Midwives Notification System (MNS), to the linked Indigenous Status Flag (ISF) derived by the Department of Health. This retrospective population-based cohort study utilised logistic regression to determine which demographic characteristics were associated with under-identification, and the effect of ascertainment method on perinatal adverse outcomes. All methods identified a core group of 19 017 (83.0%) Aboriginal women and the ISF identified 2298 (10.0%) women who were not identified using any other method. Under-ascertainment was lowest when a woman's Aboriginal status was determined by ever being recorded as Aboriginal in the MNS data, and highest when taken as it had been recorded for the birth in question. Maternal age <20 years, smoking during pregnancy, pre-existing diabetes, a history of singleton preterm birth and being in the lowest 20% of Socio-Economic Indexes for Areas score were all associated with a higher chance of being identified by the methods using only the MNS. These methods were less likely to identify nulliparous women, and those with maternal age ≥35 years. The method of ascertainment of Aboriginality did not make a significant difference to the adjusted predicted marginal probabilities of adverse perinatal outcomes. Unlinked pregnancy data can be used for epidemiological research in Aboriginal obstetric populations.

Disparities in the association between ambient temperature and preterm birth according to individual and regional characteristics: a nationwide time-stratified case-crossover study

BackgroundSeveral studies have reported that climate change elevates heat exposure in pregnant women and high temperatures during pregnancy are associated with preterm births (PTBs). Although the association might be disproportionate, related evidence remains sparse. We evaluated the disproportionate risk of PTB associated with ambient temperature during pregnancy by individual and regional characteristics in South Korea.MethodsWe collected data on birth certificates and daily mean temperatures during the period from 2011 to 2019. A time-stratified case-crossover design was used to investigate the association between temperature and PTB and stratified analyses were conducted to examine the effect modification of individual and regional characteristics.ResultsA total of 160,067 singleton PTBs were recorded in Korea from 2011 to 2019. A 5℃ increase in the mean temperature during the last four weeks before delivery was associated with an increased risk of PTB with an odds ratio (OR) of 1.03 (95% confidence interval [CI]: 1.02, 1.05), and the association was more evident in mothers aged ≥35 years (OR: 1.06 [95% CI: 1.03, 1.10]) and with low education levels (OR: 1.04 [95% CI: 1.02, 1.05]). Additionally, the estimated risk was evident in districts with lower medical resources and more prominent disparities were shown by individual and regional characteristics in rural areas than in urban areas.ConclusionsThis study provides evidence that the risk of PTB related to ambient temperature is disproportionate by individual and regional characteristics and suggests the need for public health policies to alleviate the disparities, especially in rural areas.

Perinatal and maternal outcomes in singleton preterm births of african american versus caucasian women with hypertensive disorders

Perinatal and maternal outcomes in singleton preterm births of african american versus caucasian women with hypertensive disorders

Combined effects of pre-pregnancy BMI and gestational weight gain on preterm birth: comparison between spontaneous and ART conception.

Inappropriate pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) are both linked to preterm birth (PTB); however, which one plays a dominant role in PTB risk is not yet sure. We aimed to evaluate the combined effect of pre-pregnancy BMI and GWG on the risk of PTB in singleton pregnancies conceived both spontaneously and through assisted reproductive technology (ART). The data included all mothers (n = 17,540,977) who had a live singleton birth from the US National Vital Statistics System (NVSS) 2015-2019. Logistic regression models, quantile-g-computation, and generalized additive model were used to analyze the combined association of pre-pregnancy BMI and GWG with PTB. The singleton PTB rate was significantly higher in ART pregnancies (11.5%) than in non-ART pregnancies (7.9%). When compared to those women with pre-pregnancy normal weight and GWG within Institute of Medicine (IOM) guidelines, the highest PTB risk was observed in non-ART women with pre-pregnancy underweight and GWG below IOM guidelines (aOR 2.56; 95% CI 2.53-2.60) and in ART women with pre-pregnancy obese and GWG below IOM guidelines (aOR 2.56; 95%CI 2.36-2.78). GWG dominated the combined effect with its joint effect coefficient of - 0.281 (P < 0.05) in non-ART women and - 0.108 (P < 0.05) in ART women. Inappropriate GWG played a dominant role in increasing the risk of PTB in both non-ART and ART populations. Counseling regarding pre-pregnancy BMI and especially GWG appears to be even more crucial for pregnancies conceived via ART, given their impact on PTB.

Retrospective Comparative Analysis of Neonatal Mortality and Morbidity in Preterm Singleton and Multiple Births -Single Center Experience.

Objective. To compare mortality and major neonatal morbidities between singleton preterm infants and preterm infants of multiple gestations born <33 weeks' gestation. Method. Case-control study of preterm multiples and singletons <33 weeks' born at King Abdul-Aziz Medical City Riyadh (KAMC-R) between January 2017 and December 2020. Out-born infants and infants with lethal congenital abnormalities were excluded from the study. Mortality and major neonatal morbidities including bronchopulmonary dysplasia (BPD), retinopathy of prematurity (ROP), sepsis and surgical necrotizing enterocolitis (NEC) were compared between preterm singletons and multiples. Results. A total of 803 preterm infants were included: 567 (70.6%) were singletons, 158 (19.6%) were twins and 36 (4.5%) infants were higher multiples. Adjusted mortality before hospital discharge was significantly higher among preterm infants of multiple gestations compared to preterm singletons (12.3% vs 7.9%; P = .003; AOR, 2.2; 95% CI, 1.3-3.7). Retinopathy of prematurity (ROP) needing treatment was significantly higher among preterm infants of multiple pregnancies compared to preterm singletons (11% vs 6.5%, P = .033, AOR 1.1, 95% CI, 1.04-2.99). In addition, the incidence of bronchopulmonary dysplasia (BPD) at 36 weeks post menstrual age (PMA) (29.7% vs 20.5%; P = .003; AOR, 1.7; 95% CI, 1.2-2.5) and culture positive sepsis (24.2% vs 17.5%; P = .044; AOR, 1.5; 95% CI, 1.01-2.2) were significantly higher among preterm infants of multiple pregnancy. There were no differences in mortality and adverse neonatal outcomes between twins and higher multiples. Conclusion. Preterm infants of multiple gestations suffered higher mortality and neonatal morbidities compared to preterm singleton infants despite a higher utilization of maternal antenatal steroids and better antenatal care.

Challenges in Using Progestin to Prevent Singleton Preterm Births: Current Knowledge and Clinical Advice.

Preterm birth is the leading cause of infant morbidity and mortality in children younger than 5 years old and accounts for approximately 35% of newborn deaths worldwide. The use of progestogen therapy for prevention of preterm birth has been one of the most controversial topics in modern obstetrics. Progestogens can be classified as natural or synthetic. Progesterone is a natural progestogen while progestins such as 17-alpha-hydroxyprogesterone caproate (17OHP-C) are synthetic steroid hormones. Evidence supporting the use of progestogens varies by formulation and populations studied. After more than a decade, the US Food and Drug Administration has withdrawn accelerated approval of 17OHP-C for the prevention of recurrent preterm birth in pregnant individuals with a singleton gestation. With this decision, there is no current FDA-approved treatment for prevention of spontaneous preterm birth. In this review, we provide a historical context behind the rise and fall of 17OHP-C clinical application, highlight the challenges behind the data supporting progestogen use, and offer suggestions on how to make an impact on preterm birth moving forward.

The Value of Transvaginal Ultrasonography in Management of Preterm Birth in Singletons

The Value of Transvaginal Ultrasonography in Management of Preterm Birth in Singletons

Preterm birth and stillbirth rates during COVID-19 pandemic: differences by socioeconomic status

Abstract Background At the onset of the COVID-19 pandemic, there were concerns about maternal and newborn health because of increased stress, economic difficulties and restrictions to healthcare. However, preterm birth (PTB) rates declined in some countries in 2020, while stillbirth (SB) rates remained stable or were slightly elevated. Hypotheses for the PTB result include positive consequences of lockdowns (more rest, less pollution, less exposure to infection) or restricted healthcare, leading to fewer indicated preterm births. These effects have not been examined on a population-level by socioeconomic (SES) group. Methods The Euro-Peristat network developed a Common Data Model and R scripts to produce aggregated data tables from vital statistics and birth registers. This analysis uses data from 2015 to 2020 on singleton PTB rates and SB rates as well as on mother's educational level (preferred) or area level deprivation/occupation of mother (if education unavailable). SES measures were harmonised into 3 groups (low, medium, high). We computed observed versus expected risks of the outcomes for the year 2020, given linear trends from 2015 to 2019, overall and by SES group; these estimates were pooled using random effects meta-analysis. Results 19 countries provided data on perinatal outcomes by SES group. The overall pooled mixed effect measure for PTB was -0.04 (95%CI of -0.06 to -0.02), corresponding to a 4% decline in 2020 over expected levels. This estimate was the same in low and high SES groups (-0.04 (-0.07 to 0.02) versus -0.04 (-0.06 to -0.02)). For stillbirth, the overall pooled measure was 0.02 (-0.03 to 0.07), corresponding to an increase of 2%, with higher estimates in the low SES group 0.05 (-0.02 to 0.11) versus 0.2 (-0.3 to 0.7) in the high group, although confidence limits overlapped. Conclusions A small decrease in PTB was observed in all SES groups, while results are suggestive of increases in SB rates among women in lower SES groups. Key messages • Previous results in some countries of decreased preterm birth rates in the general population in 2020 were confirmed in this European study, with similar associations in all socioeconomic groups. • Stillbirth rates were not increased in the overall population, but a 5% increase was observed in lower SES groups.

Estimating the individual singleton preterm birth risk: nomogram establishment and independent validation.

To establish and independently validate nomograms for predicting singleton preterm birth (PTB) risk based on a large sample size comprising data from two independent datasets. This cohort study used data from 50 states and the District of Columbia in the National Vital Statistics System (NVSS) database between January 2016 and December 2020. Multivariate logistic regression analysis was used to confirm the independent risk factors for PTB. Statistically significant variables were incorporated into the logistic regression models to establish PTB prediction nomograms. The models were developed using the United States (US)-derived data and were independently validated using data from US Territories. A total of 16,294,529 mother-newborn pairs from the US were included in the training set, and 54,708 mother-newborn pairs from the US Territories were included in the validation set. In all, 4 nomograms were built: 1 to predict PTB probability, and another 3 to predict moderately and late PTB probability, very PTB probability, and extremely PTB probability, respectively. Hypertensive eclampsia and infertility treatment were found to be the top 2 contributors to PTB. We developed and validated nomograms to predict the individualized probability of PTB, which could be useful to physicians for improved early identification of PTB and in making individualized clinical decisions.

P-752 Cervical cerclage versus vaginal progesterone in the prevention of preterm birth in singleton pregnancies among women with a short cervical length: A retrospective cohort study

Abstract Study question To compare the maternal and neonatal effects of cervical cerclage and vaginal progesterone in preventing singleton preterm births in women with a short cervical length. Summary answer Cervical cerclage showed no benefit in preventing preterm birth; however, it prolonged gestation by 39 days compared to vaginal progesterone treatment. What is known already Clinically, there may be a bias toward the choice of cervical cerclage if the cervix is short (&amp;lt;10 mm) and, similarly, toward vaginal progesterone use if the cervix is long (&amp;gt;30 mm). However, there are no clear treatment guidelines for the best options for singleton pregnant women with a CL between 10 and 30 mm. Furthermore, there is insufficient evidence to determine whether one treatment is superior to the other in preventing PTB and decreasing adverse neonatal outcomes in singleton pregnancies in such cases. Study design, size, duration Asymptomatic pregnant women having singleton pregnancies, with a CL between 10 and 30 mm, measured using transvaginal ultrasound at 12–26 weeks of gestation, who delivered at our hospital between January 2009 and August 2020, were included in our study. Based on the inclusion and exclusion criteria, 267 singleton pregnant women, of whom 116 were treated with cervical cerclage and 151 with vaginal progesterone, were finally included. Participants/materials, setting, methods Independent sample t-tests and differential analysis were used to analyze the data based on their distribution. The Cox semi-parametric regression model was used to analyze the gestation weeks at delivery and neonatal outcomes. The decision regarding cesarean delivery was based on a binary logistic regression model. Main results and the role of chance In the initial analysis, the number of preterm births was significantly higher in the cerclage group than in the vaginal progesterone group; however, after multivariate adjustment for confounding factors, the latency period from diagnosis to delivery was found to be significantly prolonged in the cerclage group. Limitations, reasons for caution There are currently no uniform criteria for determining whether study subjects are eligible for cervical cerclage or vaginal progesterone treatment, which may present some cases of noncompliance. Wider implications of the findings In our study, we found that cervical cerclage did not have significant advantages over vaginal progesterone treatment in preventing PTB. However, upon adjustment for confounding factors, it was shown to prolong the gestational age by 39 days compared to vaginal progesterone treatment. Trial registration number non-clinical trials

Prevalence and risk factors for preterm delivery in UNIOSUN Teaching Hospital, Osogbo - A 5 year retrospective review

Introduction: Preterm delivery is the leading cause of death in the neonatal period. It causes 28% of perinatal mortality. In Nigeria, it is responsible for 40-60% of perinatal morbidity. According to a U.S. research, preterm births have surged globally. In 2016, 16.8% of singleton live births in Lagos, Nigeria, were preterm.Methods: It was a retrospective review of patients with singleton preterm delivery in UNIOSUN Teaching Hospital, Osogbo from July 2013 to June 2018. Case records of mothers/patients with preterm deliveries were retrieved. Information on the patients' age, parity, educational status, weight, body mass index, number of antenatal visits, identifiable causes of preterm delivery and others were all extracted.Result: During the research period, 2,234 babies were born, including 210 preterm singletons out of which 147 were reviewed. Singleton preterm birth prevalence was 9.4%.Of the 147 mothers, n (20.4) had premature rupture of membrane (PROM), hypertensive disorders in pregnancy occurred in n(17.0%), Urinary Tract Infection (10.8%) and malaria in pregnancy (6.1%).Conclusion: Preterm birth rates were low compared to recent rates in the country. PROM, malaria in pregnancy, UTI in pregnancy, hypertensive disorders in pregnancy and previous history of spontaneous miscarriage were important causes/risk factors for preterm delivery.

Obstetric Indications for Progestin Therapy

Specifically, meta-analyses of randomized trials demonstrate that vaginal progesterone reduces the risk of preterm birth in selected high-risk singleton pregnancies. 17-OHPC may also reduce the risk of recurrent preterm birth in singletons. Finally, one trial suggests that vaginal progesterone may also be beneficial in improving live birth rates in singletons with prior miscarriages and early pregnancy bleeding.

Risk factors associated with preterm birth in singletons born after IVF: a national cohort study

Research questionWhat are the risk factors for prematurity other than intrauterine growth restriction in singletons after IVF? DesignData were collected from a national registry, based on an observational prospective cohort of 30,737 live births after assisted reproductive technology (fresh embryo transfers: n = 20,932 and frozen embryo transfer [FET] n = 9805) between 2014 and 2015. A population of not-small for gestational age singletons conceived after fresh embryo transfers and FET, and their parents, was selected. Data on a number of variables were collected, including type of infertility, number of oocytes retrieved and vanishing twins. ResultsPreterm birth occurred in 7.7% (n = 1607) of fresh embryo transfers and 6.2% (n = 611) of frozen–thawed embryo transfers (P < 0.0001; adjusted odds ratio [aOR] = 1.34 [1.21–1.49]). Endometriosis and vanishing twin increased the risk of preterm birth after fresh embryo transfer (P < 0.001; aOR 1.32 and 1.78, respectively). Polycystic ovaries or more than 20 oocytes retrieved also increased preterm birth risk (aOR 1.31 and 1.30; P = 0.003 and P = 0.02, respectively); large oocyte cohort (>20) was no longer associated with the risk of prematurity in FET. ConclusionEndometriosis remains a risk for prematurity even in the absence of intrauterine growth retardation, which suggests a dysimmune effect. Large oocyte cohorts obtained by stimulation, without clinical polycystic ovary syndrome diagnosed before attempts, do not affect FET outcomes, reinforcing the idea of a phenotypic difference in the clinical presentation of polycystic ovary syndrome.

Short-term effects of air pollution exposure on the risk of preterm birth in Xi’an, China

Introduction Long-term exposure to air pollution is known to be harmful to preterm birth (PTB), but little is known about the short-term effects. This study aims to quantify the short-term effect of particulate matter with aerodynamic diameter ≤ 2.5 μm (PM2.5), ≤10 μm (PM10) and nitrogen dioxide (NO2) on PTB. Materials and methods A total of 18,826 singleton PTBs were collected during the study period. Poisson regression model combined with the distributed lag non-linear model was applied to evaluate the short-term effects of PTBs and air pollutants. Results Maternal exposure to NO2 was significantly associated increased risk of PTB at Lag1 (RR: 1.025, 95%CI: 1.003–1.047). In the moving average model, maternal exposure to NO2 significantly increased the risk of PTB at Lag01 (RR: 1.029, 95%CI: 1.004–1.054). In the cumulative model, maternal exposure to NO2 significant increased the risk of PTB at Cum01 (RR:1.026, 95%CI: 1.002–1.051), Cum02 (RR: 1.030, 95%CI: 1.003–1.059), and Cum03 (RR: 1.033, 95%CI: 1.002–1.066). The effects of PM2.5, PM10 and NO2 on PTB were significant and greater in the cold season than the warm season. Conclusions Maternal exposure to NO2, PM2.5 and PM10 before delivery has a significant risk for PTB, particularly in the cold season. Key messages Maternal exposure to NO2 was significant associated with an increased risk of preterm birth at the day 1 before delivery. Particle matter (PM2.5 and PM10) showed a significant short-term effect on preterm birth in the cold season. The effects of air pollutants on preterm birth was greater in the cold season compared with the warm season.

Iatrogenic calcinosis cutis in a preterm infant.

Extravasation phlebitis over the right forearm of a preterm newborn persisted beyond 3 weeks of limb care. The initial swelling evolved into a firm, non-tender, non-erythematous induration. The newborn was born as a singleton preterm birth at 32 weeks of gestation with a birth weight of 2.1 Kg (81 percentile) due to antepartum hemorrhage in a primigravida mother. The chart review suggested intravenous calcium gluconate infusion at 80 mg/kg per day for 2 days because of low ionized calcium (0.70 mmol/L) and seizures on day 2 of life.

Effect of prior spontaneous preterm birth on delivery timing in subsequent twin pregnancy

People pregnant with twins who had a prior, singleton preterm birth are known to be at risk for preterm birth; however, prior data did not distinguish between spontaneous and iatrogenic births in the prior singleton pregnancy. We aimed to investigate the recurrence risk for preterm birth in a subsequent twin pregnancy in those who had prior spontaneous preterm birth (PSPTB) compared to those without PSPTB in a modern obstetric cohort. This is a secondary analysis of data from a retrospective cohort aimed to evaluate cell-free DNA screening performance in patients with twin pregnancies from 12/2011- 2/2020. We included twin pregnancies >20 weeks gestation and excluded those with nulliparity, unknown pregnancy outcome in twin pregnancy and unknown prior pregnancy history. Presence or absence of PSPTB was confirmed by medical record review. Baseline characteristics, gestational age of twin pregnancy and preterm birth in twin pregnancy were compared between those with history of PSPTB versus those without PSPTB. Multivariable logistic regression was used to assess adjusted odds ratio and adjust for confounders. Of the 958 twin pregnancies, 877 had no PSPTB and 81 had PSPTB. Those with twin pregnancies and prior PSPTB had a higher incidence of preterm birth (PTB) overall (77.8% vs 55.2%, OR 2.84 95%CI 1.66-4.87) and lower gestational age at delivery (34.7 vs 36.6 weeks, p< 0.001) than those with twin pregnancies without PSPTB. However, the rate of spontaneous PTB was not significantly different (46.8% vs 41.7%, OR 1.28 95% CI 0.75-2.17). For patients with PSPTB the adjusted odds ratio for PTB was 3.14 (95% CI 1.72,5.72) after controlling for Black race, body mass index, tobacco use, assisted reproductive technology, chronic hypertension and pregestational diabetes. Pregnant persons who have PSPTB are at increased risk for preterm birth in a subsequent twin pregnancy, and this information may be useful for risk counseling.View Large Image Figure ViewerDownload Hi-res image Download (PPT)

Prevalence, associated factors, and outcomes of singleton preterm births at a Tertiary Hospital in Port-Harcourt, Nigeria

Prevalence, associated factors, and outcomes of singleton preterm births at a Tertiary Hospital in Port-Harcourt, Nigeria

Risk Factors of Preterm Birth and Low Birth Weight in Singletons Conceived Through Frozen Embryo Transfer: A Retrospective Study.

The risks of adverse perinatal outcomes in offspring conceived following frozen-thawed embryo transfer (FET) assisted reproductive technology (ART) are inconsistent. The aim of this study was to analyze the risk factors for preterm birth and low birth weight in singletons after FET. 386 FET cycles was conducted at the Reproductive Medicine Center of Meizhou People's Hospital. The relationship between clinical characteristics and outcomes (term birth and preterm birth, normal birth weight and low birth weight) was analyzed. The rate of primary infertility, basal FSH and T levels, gestational age, birth weight, and proportion of male fetuses were significantly different in the preterm and full-term groups. Logistic regression analysis showed that high maternal age (≥35 years) (OR 3.652, 95% CI: 1.683-7.925, P=0.001), primary infertility (OR 2.869, 95% CI: 1.461-5.632, P=0.002), low FSH level (<6.215 mIU/mL) (OR 3.272, 95% CI: 1.743-6.144, P<0.001), and hormone replacement therapy (HRT) method (OR 2.780, 95% CI: 1.088-7.100, P=0.033) may increase risk of preterm birth after FET. Gestational age and birth weight were significantly different in fetuses with low birth weight (<2500g, n=38) and normal birth weight (≥2500g and <4000g, n=333). Logistic regression analysis showed that low basal FSH level (<6.215 mIU/mL) (OR 0.425, 95% CI: 0.209-0.865, P=0.018), and HRT method of endometrial preparation for FET (OR 0.272, 95% CI: 0.079-0.934, P=0.039) may reduce the risk of low birth weight after FET. High maternal age, primary infertility, low FSH level, HRT method of endometrial preparation for FET, and male fetus may increase risk of preterm birth after FET. In addition, primary infertility, low basal FSH level, and HRT method of endometrial preparation may reduce the risk of low birth weight after FET.