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Associations between newborn birth weight and the mother’s risk of cardiovascular disease
Normal to high birth weight in the first pregnancy, and low birth weight in the second pregnancy, is related to mother’s increased risk of dying of cardiovascular disease, a recent study shows.
The newborn birth weight is an important indicator of both the child's immediate and long-term health. However, it can also provide information about the mother's long-term health.
Previous studies have shown that mothers who give birth to children with a low birth weight (below 2,500 grams) have an increased risk of developing cardiovascular diseases. The risk is even greater if the child was born before term, so before 37 completed weeks of pregnancy.
“However, these studies have only focused on the birth weight in the first pregnancy and have not taken into account the birth weight of subsequent births,” Yeneabeba Sima says. She is a PhD candidate at the Department of Global Health and Primary Care at the University of Bergen.
Norwegian mothers often have more than one child.
“Therefore, when examining the link between newborn birth weight and the mother's mortality from cardiovascular diseases, it can be wrong to draw conclusions based only on the birth weight of the first child,” Sima explains.
Low newborn weight in both pregnancies
Sima and colleagues conducted an analysis of 753,244 mothers who were registered with two or more pregnancies in the Medical Birth Register from 1967 and followed up until 2020.
By including birth weight from both pregnancies, the researchers found that mothers' risk of dying from cardiovascular disease varied depending on the different newborn birth weight combinations.
They found that mothers who had babies with low birth weight (below the 25th percentile) in both their first and second pregnancies had a 66 per cent higher risk of dying from cardiovascular diseases before the age of 70, compared to mothers whose baby's birth weight was in the middle range (between the 25th and 75th percentile) for both pregnancies.
Percentiles are a way to split data into equal parts. When comparing all babies in Norway, newborns below the 25th percentile were those with birth weights in the lower 25 per cent, while the 75th percentile referred to babies with birth weight higher than three-quarters of the babies born in the country.
“In contrast, mothers with newborn birth weights above the 75th percentile in their first two pregnancies had no increased risk of dying from cardiovascular diseases, despite having a higher incidence of diabetes, a known risk factor for developing cardiovascular diseases later in life,” Sima says.
Important to consider birth weight in both pregnancies
The researchers also observed that mothers who gave birth to babies with low birth weight in the lower quartile in their first pregnancy, and middle or high birth weight in their second, did not have increased risk of dying of cardiovascular disease, compared to if both children had a normal birth weight.
If, however, the mother gave birth to a child with a weight in the middle or upper quartile, and later a baby in the lower quartile, the risk of dying of cardiovascular disease was increased, compared to if both the babies had a normal birth weight.
“This heterogeneity in the relationship between birth weight and the mother's mortality would not have been visible if we had only focused on birth weight in the first pregnancy,” Sima points out.
Heterogeneity refers to the presence of variations or differences within a certain group or population.
Increased risk also without pregnancy complications
Pregnancy complications, such as hypertensive disorders of pregnancy, diabetes mellitus, perinatal loss, and inability to conceive are known to affect women’s long-term cardiovascular health.
The researchers found that women who had babies with birth weights in the lower quartile during their first two pregnancies faced an increased risk of dying from cardiovascular disease, even in the absence of such complications.
“Other important factors such as diet, body mass index, smoking history, and physical activity are known to be important indicators of long-term cardiovascular health. However, these were not evaluated in this study as we did not have available data,” Sima says.
Intervening with labour did not affect the mortality risk
The study also sought to determine whether intervening during labour, specifically through induction or planned caesarean delivery, among women with term pregnancies (37 weeks or more), made changes to the result.
It did not, but Sima thinks this is because all the mothers included in the study had full-term births.
“It is most likely that this population constitutes a healthier group,” Sima says.
Should also consider subsequent newborn’s birth weight
“The newborn’s birth weight turns out to be an important factor for the mother's long-term health, but it is not sufficient to rely only on information from the mother's first birth," Sima says.
She explains that if we look at the newborn's birth weight from both the first and second pregnancies together, we can obtain valuable knowledge about the mother's chances of dying early from cardiovascular diseases.
Sima thinks that this is something clinicians should consider.
“Clinicians should take into account a woman's full reproductive history when identifying high-risk subgroups that may benefit from specific follow-up. Not considering subsequent offspring’s birth weight may be a missed opportunity,” she concludes.
Sima et al. Birth Weight in Consecutive Pregnancies and Maternal Cardiovascular Disease Mortality Among Spontaneous and Iatrogenic Term Births: A Population-Based Cohort Study, American Journal of Epidemiology, 2023. DOI: 10.1093/aje/kwad075
Rich-Edwards et al. Duration of pregnancy, even at term, predicts long-term risk of coronary heart disease and stroke mortality in women: a population-based study, American Journal of Obstetrics and Gynecology, 2015. DOI: 10.1016/j.ajog.2015.06.001
Rich-Edwards et al. Pregnancy characteristics and women's future cardiovascular health: an underused opportunity to improve women's health?, Epidemiol Rev., 2014. DOI: 10.1093/epirev/mxt006
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