Author: Kieron Barclay Does the age of the mother at the time of pregnancy and childbirth affect the health of the baby? This is a question that may intrigue many parents and potential parents in Sweden, particularly given that the average age at which mothers give birth in Sweden has increased from less than 27 in the early 1970s to around 31 today. But our recent study in American Journal of Epidemiology shows that the mother’s age is not related to factors such as premature birth or low birth weight – at least not among mothers who have at least two children.
In particular, the number of births to mothers aged 35 or older has increased substantially over the past four decades. This increase in the average age at childbearing for both women and men has actually been seen across a wide range of high-income countries in Europe, North America, and Asia since the 1970s. Given the remarkable increase in the average age at childbearing, it is important to understand how this factor may be related to child health.
In our study my colleagues and I used population register data from Finland on births between 1987 and 2000 to examine whether a mother’s age affected the risk of her child being born with low birth weight, or being born pre-term. Pre-term birth and low birth weight not only indicate poor infant health, but they have also been linked to worse outcomes in the long-term. Relative to children born with normal birth weight, or born at full gestational age (9 months), children born pre-term or with low birth weight typically have worse grades in high school, worse labor market outcomes, and worse health in adulthood.
Previous research on the relationship between maternal age and birth outcomes has shown that giving birth at older ages increases the risk of pre-term birth and low birth weight, as well as other poor pregnancy outcomes. However, we were concerned that there might be a shared common cause related to both the age at which mothers give birth, as well as the health of the children. For example, mothers who are less healthy may find it more difficult to become pregnant, on average leading to pregnancy at older ages, and maternal health may also be related to the risk of pre-term birth and low birth weight. If there is indeed a common factor that we do not observe, this could mean that it is actually not maternal age that is the problem, but some other underlying issue. In other words, older mothers may indeed have a higher rate of pre-term births and low birth weight, but his might not have anything to do with the age of the mother per se. Knowing whether maternal age itself is the problem could materially affect the decisions that potential mothers might make about when they should have children, or the advice that doctors should give.
To get around the potential problem of a shared common cause, we used a statistical technique where we compared children born to the same mother and father, i.e. full siblings. In that way, we could try to isolate maternal age from other factors, such as genetics, maternal health, and any other conditions that might potentially be related to the timing of childbearing as well as birth outcomes. By comparing full siblings, we automatically adjust for many of these shared factors, since they are common to children, siblings, who share the same mother and father. By comparing full siblings we are also able to isolate the relationship between maternal age at the time of birth and the risk of pre-term birth and low birth weight more carefully.
When we used this sibling-comparison approach, we found that maternal age is not related to the risk of pre-term birth or low birth weight in Finland, at least amongst mothers who have at least two children. Although our data shows that the children of older mothers experience pre-term birth and low birth weight at a higher rate than younger mothers, our study suggests that this pattern is not attributable to maternal age itself. These results suggest that the relationship between maternal age at the time of birth and the risk of pre-term birth and low birth weight may actually be driven by some other third factor, which remains unknown, but could be some factor such as the health of the mother. Given that our findings challenge long-held conclusions, we hope that they will spur other researchers to re-examine this question carefully, particularly in other contexts where mothers do not benefit from the excellent health system and pre-natal care available in Finland.
Reference: Alice Goisis, Hanna Remes, Kieron Barclay, Pekka Martikainen and Mikko Myrskylä, ”Advanced maternal age is not an independent risk factor for low birth weight or preterm birth”, American Journal of Epidemiology. https://doi.org/10.1093/aje/kwx177