The association between body mass index and live birth and maternal and perinatal outcomes after in-vitro fertilization: a national cohort study

Abstract

Objective: To investigate the association between female body mass index (BMI) and live birth rates and maternal and perinatal outcomes after in-vitro fertilization (IVF).

Methods: We performed a national, population-based cohort study including women undergoing IVF between 2002 and 2020. The cohort included 126,620 fresh cycles and subsequent frozen embryo transfers between 2007 and 2019 (subpopulation 1) and 58,187 singleton deliveries between 2002 and 2020 (subpopulation 2). Exposure was female BMI (kg/m²) categorized according to the World Health Organization as underweight (<18.5), normal weight (18.5-24.9, reference), overweight (25.0-29.9), class I obesity (30.0-34.9), class II obesity (35.0-39.9), and class III obesity (≥40.0). The primary outcome in subpopulation 1 was cumulative live birth per started fresh IVF cycle, including fresh and subsequent frozen embryo transfers. Primary outcomes in subpopulation 2 were hypertensive disorders of pregnancy and preterm birth at less than 37 weeks. Risk ratios (RRs) with 95% confidence intervals (CIs) for the association between BMI class and outcomes were calculated using generalized linear models after adjustment for relevant confounders.

Results: The cumulative live birth rate decreased significantly with increasing BMI from 32.6% in normal-weight women to 29.4% in overweight women, 27.0% in women in obesity class I, 21.8% in women in obesity class II, and 7.6% in women in obesity class III. The risk of hypertensive disorders of pregnancy increased significantly and progressively with increasing BMI, from 4.6% in normal-weight women to 7.8% in overweight women and 12.5%, 17.9%, and 20.3% in women in obesity classes I, II, and III. The risk of preterm birth followed a similar pattern, from 6.3% in normal-weight women to 7.5% in overweight women and 8.9%, 9.9%, and 15.3% in women in obesity classes I, II, and III. The risks of other perinatal complications, such as perinatal death, showed an even more pronounced increase.

Conclusion: Using a large and complete national cohort of women undergoing IVF, we demonstrate a dose-dependent decrease in live birth rate and a substantial increase in maternal and perinatal complications with increasing BMI. Strategies to improve this situation are warranted.

Keywords: in vitro fertilization; live birth; maternal and perinatal outcomes; obesity; population registers.

Figure 1

Live birth per first embryo transfer and cumulative live birth^† per started fresh IVF cycle in relation to BMI class. ^†Cumulative live birth rate: the number of deliveries with at least one live born child (singleton or multiple) per started fresh IVF cycle, including one fresh and/or all frozen embryo transfers within 1 year, until one delivery with a live birth or until all embryos were used, whichever occurred first. BMI, body mass index.

Figure 2

Adjusted risk ratios (95% confidence intervals) for maternal and perinatal outcomes by BMI class. Adjusted for year of treatment (continuous), maternal age (continuous), maternal country of birth (Sweden/other European/outside Europe), maternal educational level (ordinal), fertilization method (IVF/ICSI), type of embryo transfer (fresh/frozen), parity (continuous), and maternal smoking (yes/no). BMI, body mass index; CS, cesarean section; NICU, neonatal intensive care unit.