The Nurses’ Health Study II (NHSII) is an ongoing prospective cohort study of 116 430 female nurses aged 24-44 at study inception in 1989.17 The participants received a biennial questionnaire regarding disease outcomes and lifestyle behaviors, such as smoking status and medication use. Follow-up for each questionnaire cycle was greater than 90%.18
We included participants from NHSII in this analysis if they reported at least one incident singleton pregnancy that lasted more than six months. The 1991 questionnaire was the first time dietary information was captured. We therefore set this year as the baseline for this analysis and included only pregnancies occurring after the return of 1991 questionnaire. The 2001 questionnaire was the last time GDM was ascertained as most participants had passed reproductive age by then; thus, length follow-up of GDM was up to the return of the 2001 questionnaire. Individual pregnancies were eligible if the woman did not report GDM in a previous pregnancy and had no prior diagnosis of type 2 diabetes, cardiovascular disease, or cancer. We excluded pregnancies if the woman did not return at least one pre-pregnancy food frequency questionnaire, left more than 70 questionnaire items blank, or reported unrealistic total energy intake (< 2510 or > 14 644 kJ/day, equivalent to 3500 kcal/day). We excluded women with GDM in a previous pregnancy because they might change their diet and lifestyle during the next pregnancy to prevent recurrence.