The extent of placental trimming before weighing varies within and between obstetric units. Quantification of the contribution of the umbilical cord and membranes to placental weight is required to enable measurements to be compared across populations. In a sample of 50 neonates born in Southampton, placentae of liveborn singletons were weighed three times; after removing any obvious blood clots, after cutting the umbilical cord, and after stripping both the foetal and maternal membranes. The correlation between untrimmed and trimmed placental weight was 0.98. Since the combined weight of the cord and membranes increased with increasing trimmed weight (correlation=0.54), the percentage rather than absolute difference between untrimmed and trimmed weight was calculated. The median difference between untrimmed and trimmed weight was 16 per cent. No association was found between the percentage difference and infant sex, duration of gestation, birthweight, maternal age, labour onset and duration, and presentation at delivery. However, the mode of delivery had a significant effect on this difference; medians for vaginal and Caesarean section deliveries were 19 per cent and 14 per cent respectively.