Clinical implications of first-trimester ultrasound dating in singleton pregnancies obtained through in vitro fertilization

BACKGROUND: In pregnancies obtained by in-vitro fertilization (IVF) the exact day of conception is known. For that reason, IVF pregnancies are currently dated according to the day of oocytes retrieval and consequent embryo transfer. The aim of the present study is to determine whether the knowledge of the exact day of conception in IVF pregnancies is a sufficient argument against dating these pregnancies by first trimester ultrasound measurement of the crown-rump length (CRL), as it is recommended in natural conceptions. METHODS: A retrospective study was performed, including all women with singleton pregnancies conceived by IVF who underwent the first-trimester ultrasound scan for the screening of aneuploidies between January 2014 and June 2019. For each pregnancy GA was determined using two alternative methods: one based on the date of embryo transfer (GA(IVF)), and one based on ultrasound measurement of CRL (GA(US)). GA were compared to search for any discrepancy. The impact of pregnancy dating on obstetric outcome was evaluated. RESULTS: Overall, 249 women were included. Comparing GA(US) and GA(IVF), a median difference of 1 [0 – 2] days emerged (p<0.001), with GA(US) being in advance compared to GA(IVF). This discrepancy persisted when subgroups were analyzed comparing different IVF procedures (conventional IVF versus ICSI, cleavage versus blastocyst transfer, frozen versus fresh transfer). No impact of the dating method on obstetric outcomes was observed, being no differences in the rate of preterm birth or abnormal fetal growth. CONCLUSIONS: In IVF pregnancies GA(US) and GA(IVF) are not overlapping, since GA(US) is mildly greater than GA(IVF). This could be due to an anticipated ovulation and fertilization in IVF pregnancy, rather than an accelerated embryo development. For that reason, it would be appropriate to date IVF pregnancies according to GA(US), despite a known date of conception, to re-align IVF pregnancies to natural ones.