Strengths and limitations
There are strengths in our study. Due to the detailed clinical data, such as pre-pregnancy weight, weight measurements at every prenatal visit and weight measurements before delivery beyond the registry, it was possible for us to study both weight gain in different periods of pregnancy and to take the differences in types of VTE (eg, DVT as well as PE) into account. Moreover, use of weight gain z-scores instead of weight gain in kilograms helped to disentangle the effects of pregnancy weight gain on VTE from the effects of gestational duration, because GWG is highly correlated to the gestational duration. Our cohort study extends previous studies by accounting for effect modification by pre-pregnancy BMI and using a gestational age-independent measure of pregnancy weight gain.
There are also limitations in our study. The number of VTE was decreased when stratified by BMI-categories, especially among obese women. For this reason, we analyze the effect of weight gain during pregnancy on VTE in obese together with overweight women. Furthermore, the incidence of VTE might be underestimated in this study, since women who have high risks of VTE during pregnancy typically receive low-molecular-weight heparin (LMWH) and are often not switched to VTE in the postpartum period, which is inevitable.