There may be regional specialisation in structure and function across the placental surface. In Riyadh, Saudi Arabia, the length and the breadth of the placental surface at birth were highly correlated, but the breadth was more closely associated with the size of the baby. To replicate this we studied 321 pregnant Saudi women in the town of Baish. We measured the size of the newborn babies and their placentas.
The association of the length and breadth of the placental surface on the baby's body size differed in boys and girls. Among boys the breadth had a stronger association with all birth measurements except crown–heel length. This was similar to the findings in Riyadh. Placental surface length was related to crown–heel length. For each centimetre in surface length, crown–heel length increased by 0.27 cm (95% CI 0.09–0.44, p = 0.004). Among girls placental surface breadth was related to crown–heel length, whereas surface length was related to birth weight, head and thigh circumferences. For each centimetre in surface breadth, crown–heel length increased by 0.33 cm (0.13–0.53, p = 0.001).
We conclude that, within Saudi Arabia, there are both geographical and sex differences in regional specialisation across the placental surface. In the adverse circumstances of Baish, linked to the mothers' short stature, boys were smaller at birth than girls. Boys may have compensated for under-nutrition by increasing the depth of spiral artery invasion rather than by recruiting additional spiral arteries. Girls may have had more effective regional specialisation across the placental surface.