The Problem with Using the Birthweight:Placental Weight Ratio as a Measure of Placental Efficiency

Peer reviewed: 
Yes, item is peer reviewed.
Scholarly level: 
Final version published as: 

Christians, J.K., Grynspan, D. Greenwood, S.L., Dilworth, M.R. The problem with using the birthweight:placental weight ratio as a measure of placental efficiency. Placenta, Volume 68, 2018. 52-58.


Date created: 
DOI: 10.1016/j.placenta.2018.06.311


The ratio of birthweight to placental weight (BW:PW) is often used as a measure of placental efficiency in humans and animals. However, ratios have properties that are known to lead to spurious results. An alternative approach is the use of residuals from regression, which reflect whether birthweight is higher or lower than expected for a given placental weight, given the population pattern. We hypothesized that biologically meaningful measures of placental efficiency would differ between placentas with and without pathology, and between adverse and normal perinatal and postnatal outcomes.


We examined associations between measures of placental efficiency (BW:PW ratio or residuals) and placental pathologyApgar scores and infant death using National Collaborative Perinatal Project data (4645 preterm births and 28497 term births).


BW:PW ratios and residuals were significantly lower in placentas showing pathologies including signs of large infarcts or hemorrhage, although many of these differences were small. Low BW:PW ratios and residuals were also associated with low Apgar scores and increased risk of postnatal death. Whereas residuals were lower in term placentas that appeared immature by microscopic examination, the opposite was true for BW:PW ratios.


The BW:PW ratio produced an artefact whereby histologically less mature placentas at term appeared to be more “efficient” than mature placentas, illustrating a known problem with the use of ratios. For other traits, residuals generally showed differences between placentas with and without pathology that were as great as those seen with BW:PW ratios, and often showed stronger associations with adverse outcomes.



The fulltext of this paper will be available in July 2019 in keeping with the embargo policy of the journal Placenta. If you require access to the paper prior to July 2019 please email

Three supplemental figures are included in the full text.

Document type: 
Rights remain with the authors.