The incidence and timing of abnormal ultrasound findings in monochorionic-diamniotic twin pregnancies

Abstract

Monochorionic-diamniotic (MCDA) twin pregnancies impart an increased risk of complications. Current guidelines recommend routine screening ultrasounds from 16 weeks gestation onward. We sought to investigate the incidence of abnormal ultrasound findings in this population and evaluate the timing at which abnormalities were most commonly noted. This was a retrospective study of MCDA pregnancies who delivered at a single academic medical center from June 2013-December 2021. The primary outcome was the frequency of abnormal ultrasound findings. The secondary outcomes included type of abnormal ultrasound finding and gestational age at which it was recognized. Anomalies were characterized into 5 categories: structural, amniotic fluid-related, growth, placental, or TTTS. 116 MCDA pregnancies were included in our sample. 51 patients (44%) had at least one abnormal ultrasound finding documented. However, some of these patients had multiple abnormal findings, for a total of 59 abnormal ultrasound findings in our sample. 73% of these findings (43/59) were noted during the second trimester (Figure 1). All 11 cases of TTTS in our sample were noted prior to 24 weeks gestation. Of the 16 abnormal ultrasound findings in the third trimester, 12 of these (75%) were growth discordance or fetal growth restriction. Of these patients, all delivered late preterm. More than half of patients with MCDA gestations in our sample had normal routine screening ultrasounds throughout pregnancy. After the second trimester, the risk of encountering new abnormalities substantially decreased. Most findings necessitating immediate intervention and counseling by a Maternal Fetal Medicine specialist, specifically TTTS and congenital anomalies, were noted prior to 28 weeks gestation. Of third trimester findings, most were growth related and resulted in late preterm delivery. Thus, it is reasonable for patients with MCDA pregnancies to resume care with their primary obstetrician for routine surveillance after 28 weeks gestation, particularly for those patients who live far from a subspecialist.