The human yolk sac size reflects involvement in embryonic and fetal growth regulation

Introduction The human yolk sac provides the embryo with stem cells, nutrients, and gas exchange. We hypothesized that more maternal resources, reflected in body size and body composition, would condition a a larger yolk sac, ensuring resources for the growing embryo. Thus, we aimed to determine the relation between maternal size in early pregnancy and yolk sac size. Material and methods This subsidiary study was embedded in the multinational World Health Organization fetal growth project that included healthy women with a body mass index of 18‐30, reliable information of their regular last menstrual period and singleton pregnancies. Yolk sac diameter, crown‐rump length, and maternal height, weight, body mass index, and body composition were assessed before 13 weeks of gestation, and the fetal biometry was repeated during the pregnancy. Results Of 140 participants, 122 with a successful yolk sac measurement were entered in the present analysis. Maternal weight was negatively associated with the yolk sac diameter (P = 0.007) and so was maternal height (P = 0.011), fat mass (P = 0.037), and lean body mass (P = 0.018), but not body mass index (P = 0.121). Significant effects were predominantly due to the female embryos and could be traced at 24 weeks of gestation. That is, a small yolk sac : crown‐rump length ratio in early pregnancy was associated with a high fetal abdominal circumference (P