Pettker, MD; James D. Goldberg, MD; and Yasser Y. This document reflects emerging clinical and scientific advances as of the date issued and is subject to change. The information should not be construed as dictating an exclusive course of treatment or procedure to be followed.
Determining gestational age and chorionicity - Multiple Pregnancy - NCBI Bookshelf
GRADE summary of findings for choosing which fetus to use to date twin and triplet pregnancies. Evidence statement Evidence was identified for all fetal ultrasound parameters prioritised for consideration in terms of determining gestational age in twin and triplet pregnancies. All evidence came from observational studies which constitute low or very low quality evidence. With regard to whether the measurements and charts used in singletons were accurate when applied to twins and triplets, no statistically significant differences in size were found between twin and singleton pregnancies using crown—rump length very low quality evidence or biparietal diameter low quality evidence. Significant differences were reported in the head circumference of larger and smaller twins compared with singletons, although this difference did not remain significant when an average of each set of twins was used very low quality evidence. There was a significant difference between smaller twins and singletons in femur length , but the difference was not significant when comparing the larger twin or the average of each set of twins with singletons very low quality evidence.