Prenatal growth trajectories and birth outcomes after frozen–thawed extended culture embryo transfer and fresh embryo transfer: the Rotterdam Periconception Cohort

Research question: Are there differences in prenatal growth trajectories and birth outcomes between singleton pregnancies conceived after IVF treatment with frozen–thawed extended culture embryo transfer at day 5, fresh embryo transfer at day 3 or naturally conceived pregnancies? Design: From a prospective hospital-based cohort, 859 singleton pregnancies were selected, including 133 conceived after IVF with frozen–thawed embryo transfer, 276 after fresh embryo transfer, and 450 naturally conceived pregnancies. Longitudinal 3D ultrasound scans were performed at 7, 9 and 11 weeks of gestation for offline crown–rump length (CRL) and embryonic volume measurements. Second trimester estimated fetal weight was based on growth parameters obtained during the routine fetal anomaly scan at 20 weeks of gestation. Birth outcome data were collected from medical records. Results: No differences regarding embryonic growth trajectories were observed between frozen–thawed and fresh embryo transfer. Birthweight percentiles after fresh embryo transfer were lower than after frozen–thawed embryo transfer (38.0 versus 48.0; P = 0.046, respectively). The prevalence of non-iatrogenic preterm birth (PTB) was significantly lower in pregnancies resulting from fresh embryo transfer compared with frozen–thawed embryo transfer (4.7% versus 10.9%; P = 0.026, respectively). Compared with naturally conceived pregnancies, birthweight percentiles and percentage of non-iatrogenic PTB were significantly lower in pregnancies after fresh embryo transfer and gestational age at birth was significantly higher. Conclusions: This study shows that embryonic growth is comparable between singleton pregnancies conceived after fresh and frozen–thawed embryo transfer. The lower relative birthweight and PTB rate in pregnancies after fresh embryo transfer than after frozen–thawed embryo transfer and naturally conceived pregnancies warrants further investigation.