The breadth of the placental surface but not the length is associated with body size at birth

Studies of pregnancies complicated by preeclampsia led to the suggestion that the surface of the placenta is aligned along two axes, measured by its breadth and length. It was hypothesised that tissue along the breadth serves as a nutrient sensor, responding to the mother's nutritional state and fetal nutritional demands, while tissue along the length has different functions. To develop this hypothesis we measured the breadth and length of the placental surface in 401 neonates born in the King Khalid Hospital, Riyadh, Saudi Arabia, and related these measurements to the baby's body size. The breadth and length of the placental surface were highly correlated (coefficient = 0.7). Nevertheless, in a simultaneous regression with both measurements, only the breadth was associated with neonatal body size. There were strong trends of increasing birth weight, ponderal index, and the circumferences of the head, chest, abdomen and thigh with increasing placental breadth. In contrast no measurement of baby's body size was related to placental length. Birth weight increased by 125 g per cm increase in placental breadth (95% confidence interval 88 to 162, p < 0.001) but only by 20 g per cm increase in placental length (-13 to 53, p = 0.2). The corresponding figures for head circumference were 0.28 cm (0.17-0.39, p < 0.001) and 0.03 (-0.07 to 0.14, p = 0.5). The associations between placental breadth and neonatal body size were strongest if the mother's height was below the median (157 cm). The associations between a larger breadth of the placental surface and a larger baby are consistent with the hypothesis that tissue along the breadth plays a key role in nutrient transfer from mother to baby. Mothers who are short in stature are known to have lower rates of protein turnover in pregnancy. In these circumstances the ability of the placenta to transfer amino acids to the fetus may be critical.