IVF embryo and gestational sac size variation at different gestations according to embryo transfer date

Do embryo and gestational sac sizes vary at known gestational age points according to the date of embryo transfer in IVF cycles, and can embryo transfer date contribute to definitive miscarriage diagnosis?

A retrospective cohort study, within a London tertiary IVF unit between 2019 and 2021. In this unit, patients undergoing fresh or frozen IVF cycles had their embryos transferred at day 3-6 of development. Patients with a confirmed intrauterine pregnancy at first scan (<9 weeks gestation) were included. The data collected included embryo transfer date, embryo age at transfer and early pregnancy ultrasound measurements, including crown-rump length (CRL) and mean sac diameter (MSD). The end point was viability at the 11-14-week dating scan.

A total of 1161 IVF embryo transfers were included in the study; 892 (76.8%) remained viable on ultrasound at 11-14 weeks and 269 (23.2%) were non-viable. In the viable cohort at the end of the first trimester, a range of embryo and gestational sac sizes were seen at specific gestational ages. The widest variation of embryo CRL was at 49 days (1-17 mm) and at 42 days for empty MSD (7-18 mm). A gestational sac with no fetal pole, at or beyond 47 days' gestation by embryo transfer date (n = 157), had a 100% positive predictive value for miscarriage, with a 100% specificity (CI 90.1 to 100.0).

The CRL and MSD size vary significantly between pregnancies of the same gestational age by embryo transfer date. At 47 days' gestation and beyond, an empty gestational sac resulted in miscarriage. If confirmed in larger studies, this may permit women to be given an earlier definitive diagnosis of miscarriage.