Impact of placental weight and fetal/placental weight ratio Z score on fetal growth and the perinatal outcome.

Objective

To classify the infants into 9 blocks based on the deviation of both placental weight (PW) and fetal/placental weight ratio (F/P) Z score and compared the incident rate of perinatal death in each of the small for date (SFD) vs. appropriate for date (AFD) vs. heavy for date (HFD) groups.

Methods

The study population consisted of 93,034 placentas/infants from women who vaginally delivered a singleton infant. They were classified into 3 groups according to infants' weight: SFD (n=3,379), AFD (n=81,143) and HFD (n=8,512). The population was classified into 9 blocks according to the combination of i) low vs. middle vs. high placental weight (PW: a sex-, parity- and gestational-age-specific placental weight) and ii) low vs. middle vs. high F/P. In both i) and ii), ± 1.28 standard deviations in the in the Z scores was used for classifying low vs. middle vs. high, with 3x3 making 9 blocks. We then determined whether or not the perinatal death in each block differed among the three groups (SFD vs. AFD vs. HFD).

Results

(1) The proportions of 'balanced growth of placenta and infant' (appropriate PW and F/P based on Z-score) were 37.6% in the SFD group, 78.8% in the AFD group, and 51.2% in HFD group. (2) The proportion of 'inappropriately heavy placenta' in the SFD group and that of 'inappropriately light placenta' in the HFD group were 0.3 and 0.4%, respectively, a very rare phenomenon. The proportions of 'inappropriately heavy placenta' and 'inappropriately light placenta' accounted for 4.1 and 5.5% in AFD group, respectively. (3) The rates of perinatal death in those with 'balanced growth of placenta and infant' were lowest in the SFD and AFD groups.

Conclusion

By showing the fact that perinatal death was lowest in cases with balanced fetal/ placental growth, we conclude that 9-block categorization of PW and F/P based on deviation in the Z-score may be a candidate factor employable for understanding fetal and placental growth and perinatal deaths.