THE EFFECTS OF PARITY ON BIRTH-WEIGHT USING SUCCESSIVE PREGNANCIES

Objective. Accurate assessment of the difference in birthweight betwee n first and second livebirths to the same woman having excluded the ef fects of physiological factors known to affect birthweight. Design. Re trospective longitudinal observational study. Setting. Three large obs tetric units in the East Midlands of the United Kingdom. Subjects and methods. Women in whom data were recorded for their first two pregnanc ies on the UK East Midlands Obstetric database which resulted in the d elivery of a liveborn, singleton and congenitally normal baby. Six tho usand five hundred and thirty such cases were identified, of which 345 7 had complete datasets and delivered both babies at term (259 to 300 days). An analysis was performed of changes between the paired pregnan cies of physiological factors known to affect birthweight. Regression analyses were used to enable prediction of the second birthweight with the knowledge of the first birthweight. Results. The mean crude birth weight difference between first and second pregnancies was an increase of 138 g. Significant differences between the paired pregnancies were found in maternal booking visit weight, blood pressure, maternal age and gestation at delivery. Independent factors affecting difference in birthweight were gestation at delivery, maternal booking weight and b aby's sex. Regression towards the mean was demonstrated which meant th at a woman delivering a first baby weighing more than 3720 g could exp ect a lighter baby for her second delivery provided that all other fac tors remained constant. Conclusions. In general terms a woman is more likely to deliver a heavier baby in her second pregnancy than in her f irst pregnancy. However, maternal physiological factors differ in the two pregnancies and these differences have additional effects on birth weight. The effects of both these observations are tempered by regress ion towards the mean which has a profound influence in predicting the likely change in birthweight between first and second pregnancies. Cli nical decisions should not be based on the assumption that a second ba by will inevitably be heavier than the first baby.