Background Pre-pregnancy weight is of interest both clinically, with concern for women entering pregnancy over- or under-weight, and in research into its effects on mother and child. It is also a pre-requisite for assessment of pregnancy weight gain, also of interest clinically and in research. However, few women have clinical or research measurements of weight taken before pregnancy. Two methods are used: women’s recall of their pre-pregnancy weight; and an actual measure of weight taken in early pregnancy. The Southampton Women’s Survey (SWS) allows a comparison of these methods against measured pre-pregnancy weights.

Methods The SWS is a mother and child cohort of 12,583 women aged 20–34 years who were recruited when not pregnant and of whom 3,158 were followed through a subsequent pregnancy. 198 women with an estimated date of conception within three months of their recruitment interview form the analysis sample. The ‘gold standard’ pre-pregnancy weight measure was the woman’s weight at the recruitment interview. Two ‘proxy’ measurements were compared with the ‘gold standard’, namely: (1) the woman’s recall of her pre-pregnancy weight during early pregnancy and (2) measured weight in early pregnancy (around 11 weeks’ gestation). Bland Altman plots were used for the comparison. Sensitivity analyses were conducted of women who became pregnant within one and six months of the recruitment interview.

Results The medians (interquartile ranges (IQR)) of the time between the women’s recruitment interview and conception, and the gestational age at early pregnancy measurement were 7.4 (3.7–9.6) and 11.7 (11.3–12.3) weeks respectively. Recalled weights were a median of 1.40 kg (IQR: 0.10 to 3.2 kg) lighter than the measured pre-pregnancy weights, while the measured weights in early pregnancy were 0.80 kg heavier (IQR: −0.70 to 2.5 kg). The limits of agreement for recalled weights were −7.59 to 4.29 kg, which were wider than for the early pregnancy measured weights: −3.71 to 5.47 kg. In the sensitivity analyses, the limits of agreement were slightly narrower for the early pregnancy measures when considering the 61 women who conceived within one month of recruitment interview (−3.14 to 4.92 kg) but wider for the 371 who conceived within six months (−4.53 to 6.24 kg); the limits of agreement were similar for the recalled weights in all three datasets.

Conclusion These findings suggest that a measured weight in early pregnancy is a better assessment of pre-pregnancy weight than recalled weight, indicating that, where available, early pregnancy measures can replace recalled weights in clinical practice and research.