To compare first-trimester fetal crown-rump length (CRL) measurements in pregnancies obtained after thawed blastocyst transfer versus fresh blastocyst transfer after in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI).

Prospective longitudinal cohort study of CRL Z scores with adjustment for major confounders.

University-affiliated obstetrics, fetal medicine, and fertility units.

Singleton gestations conceived via IVF/ICSI and fresh or thawed blastocyst transfer with ultrasound performed at 6–14 weeks of gestational age.

None.

CRL Z scores.

A total of 365 IVF/ICSI pregnancies were recruited (fresh: 161; thawed: 204). The mean CRL Z score at 6–14 weeks was significantly greater in thawed versus fresh transfers. Different growth trajectories between thawed and fresh transfers were detected: Mean CRL Z score was 0 at 65 days in fresh versus 80 days in frozen. Comparisons of both fresh and thawed transfers with reference values from the general population confirmed significantly lower CRL Z scores in both IVF/ICSI groups (P<.001). The risks of CRL <5th percentile in fresh versus thawed were, respectively 68% vs. 40% at 6 weeks and 2% vs. 1% at 14 weeks. A significant positive correlation between CRL Z scores and birth weight Z scores was found only for fresh transfers, not for thawed.

At 6–14 weeks, thawed blastocyst transfers after IVF/ICSI conceptions present greater CRLs compared with fresh, and both IVF/ICSI groups show smaller CRLs than the general population. This effect is particularly evident before 9 weeks and it may favor birth weight difference of thawed versus fresh BT pregnancies.