Difference in Z scores of placental weight and fetal/placental weight ratio by mode of delivery

Aim The difference of placental weight (PW) and fetal/placental weight ratio (F/P) Z scores by mode of delivery is unclear. To investigate such differences and the actual conditions underlying the imbalance between fetal and placental growth. Methods The data from Japanese database 2013 were assessed. Light‐for‐dates (LFD, n = 12 884), appropriate‐for‐dates (n = 114 464) and heavy‐for‐dates (n = 13 164) from 140 512 placentas/infants of mothers delivered a singleton infant. Using Z scores of PW and F/P based on the standard curves of a sex‐, parity‐ and gestational‐age‐specific PW and F/P, the rate of inappropriately heavy placenta according to the mode of delivery (vaginal [VD] vs cesarean [CS]) was investigated. Results (i) The PW and F/P were heavier and bigger in VD than in CS, in each subgroup. In the LFD groups, the PW Z score in VD was higher than that in CS, whereas the F/P Z score was lower than in VD than that in CS. (ii) Data of single regression analyses between the PW Z score and F/P Z score in VD groups were different from those in CS, especially in LFD infants. (iii) In the LFD subgroups, the rates of inappropriately heavy placenta in VD (n = 7781) and CS (n = 5103) were 0.54% and 0.86%, respectively. Conclusion Difference in the mode of delivery influenced the PW and F/P, and the rate of inappropriately heavy placenta is associated with mode of delivery among LFD infants. This methodology might give us a clue to search a useful way for identifying the high‐risk groups requiring postnatal counseling.