First trimester ultrasound estimation of gestational age in pregnancies conceived after in vitro fertilization.

OBJECTIVE: To evaluate BPD as an alternative to CRL for the estimation of gestation age in late first trimester pregnancies (between 9th and 13th gestational weeks), and to construct a first trimester reference chart of fetal BPD growth.

STUDY DESIGN: A prospective study that compared the gestational age estimated by BPD and CRL with the IVF gestational age in 167 first trimester pregnancies (127 singletons, 40 twins).

RESULTS: Both BPD and CRL correlated well with the IVF gestational age (GA) from 9th to 13th gestation weeks. When comparing the difference of the GA (in days) estimated from the two respective ultrasound parameters versus the GA based on IVF (oocyte retrieval day +14 days), BPD estimations had a significantly lower mean difference than CRL (0.013 vs. 0.746; p<0.01), as well as a lower standard deviation (2.414 vs. 3.008; p<0.05). In addition, the 95% limits of agreement between the BPD estimated GA and IVF GA were also smaller than CRL estimated GA versus IVF GA (-4.719 to 4.745 vs. -5.149 to 6.641).

CONCLUSION: Biparietal diameter shares similar accuracy with crown rump length in late first trimester ultrasound estimation, with additional advantages of lower random measurement errors.