On an Anatomical Basis for the Increase in Birth Weight in Second and Subsequent Born Children

Objective. Structural changes of the spiral arteries needed for the pregnancy do not completely resolve following parturition. We tested the hypothesis that these anatomical changes are related to the number of previous pregnancies. Methods. Video image analysis was performed on Masson-stained sections from 28 hysterectomy specimens, examining the proportion of non-muscular tissue within the spiral arterial walls. Semi-quantitative analysis of changes of the internal elastic lamina was performed on orcein-stained sections. The extent of changes was tested for correlation with parity using regression analysis. Results. Duplication and fragmentation of the internal elastic lamina increased with increasing parity (P=0.019) with differences between nulliparity and parity 1, 2, 3 or 4 but not between other parity pairings. The proportion of non-muscular tissue increased with increasing parity (P=0.010). Conclusions. These results suggest that pregnancy results in permanent anatomical changes in the spiral arteries that may modify subsequent vascular remodelling in the next pregnancy. This could provide an anatomical basis to explain the epidemiological data showing an increasing birth weight with increasing parity, particularly of the second born.