Rate of gestational weight gain, pre-pregnancy body mass index and preterm birth subtypes: a retrospective cohort study from Peru

Abstract

Objective: To examine the shape (functional form) of the association between the rate of gestational weight gain, pre-pregnancy body mass index (BMI), and preterm birth and its subtypes.

Design: Retrospective cohort study.

Setting: National reference obstetric centre in Lima, Peru.

Population: Pregnant women who delivered singleton babies during the period 2006-2009, resident in Lima, and beginning prenatal care at ≤ 12 weeks of gestation (n=8964).

Methods: Data were collected from the centre database. The main analyses consisted of logistic regression with fractional polynomial modelling.

Main outcome measures: Preterm birth and its subtypes.

Results: Preterm birth occurred in 12.2% of women, being mostly idiopathic (85.7%). The rate of gestational weight gain was independently associated with preterm birth, and the shape of this association varied by pre-pregnancy BMI. In women who were underweight, the association was linear (per 0.1 kg/week increase) and protective (OR 0.88; 95% CI 0.82-1.00). In women of normal weight or who were overweight, the association was U-shaped: the odds of delivering preterm increased exponentially with rates <0.10 or >0.66 kg/week, and <0.04 or >0.50 kg/week, respectively. In women who were obese, the association was linear, but non-significant (OR 1.01; 95% CI 0.95-1.06). The association described for preterm birth closely resembled that of idiopathic preterm birth, although the latter was stronger. The rate of gestational weight gain was not associated with indicated preterm birth or preterm prelabour rupture of membranes.

Conclusions: In Peruvian pregnant women starting prenatal care at ≤ 12 weeks of gestation, the rate of gestational weight gain is independently associated with preterm birth, mainly because of its association with idiopathic preterm birth, and the shape of both associations varies by pre-pregnancy BMI.

Figure 1

Selection of women included in the study. The percentages refer to the total number of women who delivered in the Instituto Nacional Materno-Perinatal (INMP) in Lima, Peru, during 2006–2009 (n = 66 630). †Comorbidities excluded from the study: tuberculosis (n = 256, 0.4%), human immunodeficiency virus (HIV) infection (n = 338, 0.5%), and congenital malformations (n = 52, 0.1%).

Figure 2

Adjusted odds ratios (AORs) of preterm birth (all subtypes) and 95% confidence intervals (95% CIs) by rate of gestational weight gain (kg/week) and pre-pregnancy body mass index categories; AORs, solid lines; 95% CIs, broken lines. The natural logarithm of the odds of preterm birth was calculated from the following models: (A) underweight, ln y = −18.838 − 0.077(rate + 1) − 0.144(BMI) + 4.444(PCV)⁻¹ + 2.072(PCV)⁻¹ ln(PCV) − 0.055(GA) − 0.075(parity + 1) + 0.014(age) + 16.994(SES2) + 17.061(SES3) + 0.653(HTN); (A, B) normal weight, ln y = −2.235 − 2.2885(rate + 1)^½ + 0.204(rate + 1)³ − 0.019(BMI) + 9.270 ln(PCV) − 26.780(PCV)^½ − 0.049(GA) − 0.003(parity + 1)⁻² + 0.678(age)⁻¹ + 0.266(SES2) + 0.348(SES3) + 0.431(HTN); (C) overweight, ln y = −2.745 + 1.753(rate + 1)⁻¹ + 0.195(rate + 1)³ − 0.056(BMI) + 20.657(PCV)^−½ + 7.322(PCV)^−½ ln(PCV) − 0.051(GA) + 0.080(parity + 1) + 0.009(age) + 0.573(SES2) + 0.621(SES3) + 0.287(HTN); (D) obese, ln y = −2.133 + 0.022 (rate + 1) + 0.022(BMI) − 0.026(PCV)³ − 0.045(GA) + 0.052(parity + 1) − 0.002(age) + 0.424(SES2) + 0.394(SES3) + 0.342(HTN), where y is the odds of preterm birth, rate is the rate of gestational weight gain (kg/week), BMI is the body mass index (kg/m²), GA is the gestational age at the start of prenatal care (weeks), PCV is the number of prenatal care visits, SES2 is medium socio-economic status, SES3 is high socio-economic status, and HTN is history of gestational hypertension. The reference weight gain rate during pregnancy was 0.38 kg/week (mean).

Figure 3

Adjusted odds ratios (AORs) of idiopathic preterm birth and 95% confidence intervals (95% CIs) by rate of gestational weight gain (kg/week) and pre-pregnancy body mass index categories; AORs, solid lines; 95% CIs, broken lines. The natural logarithm of the odds of idiopathic preterm birth was calculated from the following models: (A) underweight, ln y = −19.056 − 1.242(rate + 1) − 0.152(BMI) − 0.337(PCV)⁻² + 2.264(PCV)⁻¹ − 0.044(GA) − 0.349(parity + 1) + 0.007(age) + 16.980(SES2) + 17.001(SES3) + 0.825(HTN); (B) normal weight, ln y = −2.432 − 1.188(rate + 1)² + 0.538(rate + 1)³ − 0.015(BMI) + 9.751 ln(PCV) − 27.905(PCV)^½ − 0.049(GA) − 0.003(parity + 1)⁻² + 1.107(age)⁻² + 0.295(SES2) + 0.347(SES3) + 0.608(HTN); (C) overweight, ln y = −2.780 + 2.142(rate + 1)⁻¹ + 0.225(rate + 1)³ − 0.063(BMI) − 12.636(PCV)^½ − 9.498 ln(PCV) − 0.051(GA) + 0.077(parity + 1) + 0.005 (age) + 0.417(SES2) + 0.435(SES3) + 0.482(HTN); (D) obese, ln y = −2.384 − 0.101(rate + 1) − 0.014(BMI) − 1.305(PCV)³ − 0.040(GA) + 0.071(parity + 1) − 0.002(age) + 0.216(SES2) + 0.209(SES3) + 0.584(HTN), where y is the odds of idiopathic preterm birth, rate is the rate of gestational weight gain (kg/week), BMI is the body mass index (kg/m²), GA is the gestational age at the start of prenatal care (weeks), PCV is the number of prenatal care visits, SES2 is medium socio-economic status, SES3 is high socio-economic status, and HTN is a history of gestational hypertension. The reference weight gain rate during pregnancy was 0.38 kg/week (mean).