Is ideal BMI before pregnancy safe during pregnancy?

What does the evidence say about ideal BMI before pregnancy during pregnancy?

1. The standard prepregnancy BMI categories are under 18.5 for underweight, 18.5 to 24.9 for normal weight, 25 to 29.9 for overweight, and 30 or above for obesity. 1
2. In a cohort of over 50,000 couples, fecundability, meaning the monthly chance of conceiving, decreased as female BMI rose above 22, and women with pre-pregnancy overweight or obesity had a longer average time to pregnancy than women at a healthy weight. 2
3. Obese women were more than six times as likely to develop gestational diabetes as women at a healthy weight in one study (11.8 percent versus 1.9 percent), and hypertensive disorders were also markedly more common. 3
4. In an analysis of over 950,000 births, the risk of preterm birth rose with the severity of prepregnancy underweight, from about a 22 percent higher risk with mild underweight to a 61 percent higher risk with severe underweight, compared with a healthy starting weight. 4
5. Losing at least 10 pounds between pregnancies was linked to a lower risk of gestational diabetes in one study, though a larger BMI change wasn't consistently linked to a lower gestational-diabetes risk across all the studies reviewed, a reminder the evidence on weight change itself is still mixed. 1

Is ideal BMI before pregnancy safe in each trimester?

• First trimester. Early in trying to conceive is a good time to check where your BMI falls, since a large cohort study found fecundability, the monthly chance of conceiving, started declining as female BMI rose above about 22, well before reaching the overweight cutoff.
• Second trimester. If conception is taking longer than expected, both very high and very low BMI have been linked to lower monthly conception chances, so this is worth raising with a provider alongside other causes being ruled out.
• Third trimester. Once pregnant, pre-pregnancy BMI still matters: obesity before pregnancy has been linked to a much higher rate of gestational diabetes and hypertensive disorders, while underweight before pregnancy has been linked to a higher rate of preterm birth, so the goal is a healthy starting point rather than a scramble to change BMI once pregnancy is confirmed.