Perinatal morbidity and in vitro fertilization (IVF): does fresh vs. frozen embryo transfer matter?

Abstract

To explore if there is a difference in the risk of adverse perinatal outcomes in IVF pregnancies conceived after fresh as compared to frozen embryo transfer. Retrospective cohort. The underlying cohort consisted of all pregnancies conceived following transfer of fresh embryos in which two pronuclear stage (2PN) sibling embryos were available for cryopreservation and all pregnancies conceived following transfer of frozen/thawed 2PN embryos at a university center from January 1998–June 2006. The primary outcome was a composite consisting of at least one: preterm delivery (delivery <37 weeks), intrauterine growth restriction (<10th percentile), and/or low birth weight (<2500 grams). Secondary outcomes included subtypes of pregnancy loss. Associations were assessed using both univariable and multivariable logistic regression. The final sample of 340 pregnancies consisted of 218 following fresh and 122 following frozen embryo transfers. There were significantly more embryos transferred during a frozen cycle as compared to a fresh cycle (2.90 vs. 2.68, P=0.002). Despite this, frozen embryo transfer was significantly less likely than fresh to result in a twin pregnancy (OR 0.38, 95% CI 0.20, 0.70, P=0.001), and, conversely, more likely to result in a singleton (OR 2.64, 95% CI 1.56, 5.53). Age, embryo transfer type and twin gestation were selected for inclusion in the final model. First trimester pregnancy loss (biochemical loss, ectopic pregnancy, clinical pregnancy loss) was significantly reduced following frozen embryo transfer (OR 0.53, 95% CI 0.30, 0.94, P=0.030). As expected, increasing age was significantly associated with clinical pregnancy loss (first trimester loss after ultrasound confirmation of gestational sac) (OR 1.19, 95% CI 1.06,1.33, P=0.003) and twin gestation resulted in an almost 24-fold increase in risk of composite adverse outcome (OR 23.97, 95% CI 11.23, 51.15, P<0.001). This data suggests that freezing and thawing an embryo is not associated with any difference in perinatal morbidity as compared to fresh embryo transfer, and that the hormonal milieu at the time of conception may play a role in early pregnancy loss, as the risk of first trimester loss after frozen embryo transfer was nearly half that of fresh embryo transfer. Frozen embryo transfer should be utilized to minimize the number of embryos transferred during a fresh cycle, decrease the risk of multiple gestation, and, thereby, reduce the major cause of morbidity in IVF pregnancies.