Short inter-pregnancy interval and birthweight: a reappraisal based on a follow-up study of all women in Norway with two singleton deliveries during 1970-2019

Abstract

We studied mean changes in birthweight from the first to the second delivery according to length of the inter-pregnancy interval. We also studied recurrence risk of low birthweight, preterm birth and perinatal death. We followed all women in Norway from their first to their second singleton delivery at gestational week 22 or beyond during the years 1970-2019, a total of 654 100 women. Data were obtained from the Medical Birth Registry of Norway. Mean birthweight increased from the first to the second delivery, and the increase was highest in pregnancies conceived < 6 months after the first delivery; adjusted mean birthweight increase 227 g (g) (95% CI; 219-236 g), 90 g higher than in pregnancies conceived 6-11 months after the first delivery (137 g (95% CI; 130-144 g)). After exclusion of women with a first stillbirth, the mean increase in birthweight at inter-pregnancy interval < 6 months was attenuated (152 g, 95% CI; 143-160 g), but remained higher than at longer inter-pregnancy intervals. This finding was particularly prominent in women > 35 years (218 g, 95% CI; 139 -298 g). In women with a first live born infant weighing < 2500 g, mean birthweight increased by around 1000 g to the second delivery, and the increase was most prominent at < 6 months inter-pregnancy interval. We found increased recurrence risk of preterm birth at inter-pregnancy interval < 6 months, but no increased recurrence risk of low birthweight, small for gestational age infant or perinatal death. In conclusion, we found the highest mean increase in birthweight when the inter-pregnancy interval was short. Our results do not generally discourage short pregnancy intervals.

Keywords: Birthweight; Cohort study; Gestational age; Inter-pregnancy interval; Pregnancy; Recurrence risk; Risk factors.

Fig. 1

Increase in mean birthweight in grams (with 95% confidence intervals) according to the length of the inter-pregnancy interval. All women in Norway with two singleton deliveries during the period 1970-2019 and first delivery <2010. Note Crude estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles). Adjusted estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles), women’s age at the first delivery, diabetes in first or second pregnancy, hypertension in first or second pregnancy, sex of child in first and second pregnancy, a new father to the second pregnancy and whether the mother was born in Norway.

Fig. 2

Increase in mean birthweight in grams according to inter-pregnancy interval and women’s age at first delivery. All women in Norway with two singleton deliveries during the period 1970-2019 and first delivery <2010 First stillbirth excluded. Note Crude estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles). Adjusted estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles), mother’s age at the first delivery, diabetes in first or second pregnancy, hypertension in first or second pregnancy, sex of child in first and second pregnancy, a new father to the second pregnancy and whether the mother was born in Norway. Intervals up to 72 months for mothers aged >35 years

Fig. 3

Increase in mean birthweight in grams according to inter-pregnancy interval. Birthweight at first delivery <2500 g. Two singleton deliveries during the period 1970-2019 and first delivery <2010 (n=25 369). First stillbirth excluded. Note Crude estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles). Adjusted estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles), women’s age at the first delivery, diabetes in first or second pregnancy, hypertension in first or second pregnancy, sex of child in first and second pregnancy, a new father to the second pregnancy and whether the mother was born in Norway

Fig. 4

Increase in mean gestational age at delivery (in days) according to inter-pregnancy interval. All women in Norway with two singleton deliveries during the period 1970-2019 and first delivery <2010. First stillbirth excludeds. Note Crude estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles).Adjusted estimates: Include fixed effect for year of first delivery in five periods with equal number of women (quintiles), women’s age at the first delivery, diabetes in first or second pregnancy, hypertension in first or second pregnancy, sex of child in first and second pregnancy, a new father to the second pregnancy and whether the woman was born in Norway