Regular weighing of pregnant women: a relic from a bygone era

The practice of regularly weighing women during pregnancy dates to the 1940s in a strategy introduced by the government of the UK to monitor the effect of post-World War II food rationing on the maintenance of a healthy pregnancy [(1)]. This practice continued throughout the 1970s and into the 1990s to aid detection of both preeclampsia and infants who are born small for gestational age (SGA), although it was recognized to have a low positive predictive value [(2)]. There has since been a move in some countries [(3)] to recommend that a woman be weighed only at the time of the booking obstetric visit in the first trimester and then again at 36 weeks' gestation. However, this practice is not universal, with other countries adopting a more intensive approach with regular weighing at each antenatal appointment and adherence to the Institute of Medicine (now National Academy of Medicine) recommendations to limit gestational weight gain (GWG), with the expectation of improved pregnancy and birth outcomes [(4)].

In this edition, Boone-Heinonen and colleagues [(5)] report a cohort study using electronic health record data from 77,599 pregnancies between 2004 and 2020 from women whose body mass index (BMI) was greater than 18.5 kg/m². GWG was measured in each trimester of pregnancy to evaluate relationships between rate of gain and the occurrence of infant size at birth, defined as large for gestational age (LGA; >90th percentile) and SGA (<10th percentile). The authors identified the risk of birth of an infant with LGA to be greatest among women with a higher prepregnancy BMI and among those whose GWG was higher. Conversely, birth of an infant with SGA was seen in women with a lower prepregnancy BMI and lower GWG. The authors suggest that first-trimester GWG may influence birth size via an effect on second- and third-trimester weight gain and that targeting first-trimester weight gain may be a strategy for intervention.

We would like to propose an alternate view that challenges some of the widely held assumptions around GWG and its relationship, often presumed to be causal, with pregnancy and birth outcomes. Recent reports [(6-8)] have identified the lack of evidence to suggest that the effect of maternal BMI on pregnancy outcomes is mediated via an effect on GWG. That is, GWG is not on a causal pathway between maternal BMI and adverse pregnancy outcomes [(6)], with limited predictive value in the identification of women who experience a range of pregnancy and birth complications [(7, 8)].

The validity of the purported relationships between total or trimester-specific GWG and fetal size is problematic for reasons that should be immediately apparent in considering the nature of GWG as an outcome. Although GWG is a clinically simple measure to obtain, it represents a composite of maternal fat deposition; pregnancy-related plasma volume expansion; breast and uterine tissue hypertrophy; extracellular fluid; and the products of pregnancy, including the placenta, fetus, and amniotic fluid. Furthermore, the precise contribution from each of these components in an individual woman is extremely difficult to measure with any degree of accuracy. Be that as it may, any relationship between GWG and birth weight cannot be a causal relationship by definition because fetal weight is a component of GWG. This means that, necessarily, if two women are equal in all other aspects of each of the GWG components as outlined earlier, but the first fetus weighs more than the second fetus (including weight for gestational age), the first woman will necessarily have greater GWG. The only possible interpretation of the relationship between GWG and infant birth size is that women with a larger fetus give birth to larger babies.

Furthermore, the lack of a causal association between maternal BMI and GWG not only suggests that GWG is not an appropriate target to improve pregnancy outcomes but also potentially explains the very limited clinical impact of antenatal pregnancy interventions that have been observed to date. In contrast, maternal prepregnancy BMI has been identified to be causally related to adverse pregnancy outcomes [(6, 8)], with the risk increasing with increasing BMI [(6)]. This suggests that preconception interventions may be a more appropriate target for intervention to improve pregnancy outcomes for women and their infants.

An ongoing and relentless search for the “right” antenatal intervention targeting GWG as being on a causal pathway to improved clinical pregnancy and birth outcomes would seem to be an exercise in futility. Not only does such an approach unnecessarily waste limited health care resources, but it also sets women up to fail in their inability to meet unattainable goals. Rather than continuing to focus on GWG, researchers and clinicians should reconsider the validity of such an approach and the practice of actively managing GWG throughout pregnancy.

The authors declared no conflict of interest.