Thursday December 17 2015
Avoid alcohol and smoking if you are planning for a pregnancy
"Delaying motherhood is 'better for your health'," the Daily Mail says, reporting that "Women who give birth after the age of 25 'are healthier by the time they’re 40'". However, the picture is a lot more ambiguous than the paper presents.
Researchers aimed to look at whether the age a woman had her first baby was linked to her self-reported health at age 40. The study included almost 4,000 US women who were recruited in 1979 when they were aged 14 to 22, and had their first babies at an average age of 23.7. At age 40 it asked them just a single question: "In general, would you say your health is excellent, very good, good, fair, or poor?" It then looked at how this answer was linked to the age they had their first baby, in particular looking at the influence of ethnicity and marital status.
The results give a mixed and confusing picture. It found that having your first baby between 20 and 24 was linked to poorer reported health than having your baby aged 25 to 35. But having a baby aged under 20 was only associated with poorer health for a black women.
An important problem with this study though, is that age at first birth and life circumstances surrounding motherhood today are considerably different than they were 30 years ago. Not to mention that the single question used doesn’t really tell you much at all about that person’s health. Overall, this study does not provide good evidence that delaying motherhood is better for your health.
Where did the story come from?
The study was carried out by researchers from Ohio State University, Cornell University, the University of Wisconsin, and University of Akron, which are all in the US. It was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
The study was published in the peer-reviewed Journal of Health and Social Behaviour.
The Mail’s reporting would benefit from highlighting the important limitations of this study, particularly limited applicability to people today.
What kind of research was this?
This was a cohort study that aimed to examine the association between the age a woman has her first baby and her self-reported health at age 40.
The researchers discuss previous theories that earlier childbearing could put someone at a disadvantage throughout life – for example, by curtailing their educational or work opportunities and placing them at socioeconomic disadvantage. However, there are also theories that it could have negative effects on a woman’s health as well.
The current study uses data from a long-term follow-up study of women who "came of age", as the study says, in the late 1970s to see whether there is a link between age at childbirth and midlife health. They further aimed to explore the effect of marital status at time of childbirth and subsequently, to see how the links differ for people of different ethnicity.
A cohort study such as this can demonstrate associations, but it cannot prove direct cause and effect, as many other confounding factors may be involved in the link.
What did the research involve?
This study used data from the US National Longitudinal Survey of Youth 1979, which in 1979 is said to have recruited a nationally representative sample of 4,926 women aged 14 to 22 (along with a similar number of men). Participants were assessed by questionnaire every year up to 1994 and two-yearly thereafter.
This analysis includes a final sample of 3,348 women who had a first birth from the ages of 15 and 35, either while married or never married (they excluded divorced women), and who had complete questionnaire health data at age 40.
Self-assessed health was measured at age 40 by a single question: "In general, would you say your health is excellent, very good, good, fair, or poor?" – with a response ranging from 1 (poor) to 5 (best health).
The researchers looked for associations between age at first birth and self-assessed health. Being aged 25 to 35 at first birth was the reference group to which younger ages of adolescent (15 to 19) or early adulthood birth (20 to 24) were compared.
They also looked for whether this was influenced by marital status (both at first birth and subsequently). Their statistical models also took into account a number of other variables:
- existing health problems before first birth that would limit ability to work
- where the girl/woman was living at age 14 (e.g. with parents, in an urban area)
- religious affiliation
- various proxy measures of socioeconomic status (e.g. educational level and occupation of the woman’s own parent(s))
What were the basic results?
Average age at first birth was 23.7 years – said to be the national average in 1985 – with just under a quarter of births to unmarried women.
The researchers found that first births in adolescence (15 to 19) and young adulthood (20 to 24) were associated with poorer self-rated health at age 40 in women of white and black ethnicity, but not for Hispanic women.
However, they found that this link was largely influenced by marital status. Most white adolescents having their first baby were unmarried, and when this was taken into account, the significant link was removed. For black adolescent women, and for young adult white and black women, being unmarried accounted for some of the link, but did not completely remove it.
Overall, this is interpreted that first birth in young adulthood is independently associated with poorer midlife health for both black and white women, but first birth in adolescence is associated with poorer midlife health only in black women.
The researchers’ analyses further looked at the effect of marital status. Overall, it seems that women who had an earlier birth while unmarried had poorer midlife health than their counterparts who had a birth at a similar age, but were married at the time. However, this risk seemed mainly restricted to those who subsequently married. Those who remained unmarried didn’t seem to be at higher risk.
How did the researchers interpret the results?
The researchers conclude: "Findings suggest that adolescent childbearing is associated with worse midlife health compared to later births for black women, but not for white women. Yet, we find no evidence of health advantages of delaying first births from adolescence to young adulthood for either group. Births in young adulthood are linked to worse health than later births among both black and white women. Our results also indicate that marriage following a non-marital adolescent or young adult first birth is associated with modestly worse self-assessed health compared to remaining unmarried."
This study has explored the link between a woman’s age at her first child birth and her self-rated health at 40.
The researchers carried out fairly complex statistical models that give quite a confusing picture, which is difficult to take meaningful interpretation from – particularly when you are taking into account the differing effects of marital status and ethnicity.
For example, are you supposed to conclude from this that if a woman has a baby before the age of 20, she is better off remaining unmarried because it will be better for her health when she’s 40? Or that having a baby before 20 won’t adversely affect the health of a white woman, but it will for a black woman?
There are many important limitations to this study, which does not provide good evidence that delaying first birth for a woman is better for her health.
The study has limited applicability to woman having children today. The women in this study were aged 14 to 22 in 1979, and most had their first babies during the 80s, when the average age at first birth was only 23.7. The comparison group in this study – the "older" mothers – were those having their first baby from the ages of 25 to 35.
This would certainly not seem old today, when the average age at first birth is now inside this age bracket, not in young adulthood as it was then. At the time of this cohort, there was no older category of women having births from the ages of 35 upwards into their 40s, which would have been rarer in the 80s, but is far more common today.
Similarly, the strong associations of this study with marital status cannot be easily applied today. In the 80s, many more women having babies were married, and the thought that couples expecting a baby should be married was still fairly commonplace. Things are not the same now, when it is usual for people to have children in a range of different relationship circumstances. Also, the researchers’ theories of how childbirth may lead to socioeconomic disadvantage though curtailing educational and work opportunities are also likely to be less relevant today than they were in the 80s, as many more mothers work today.
Another important limitation is the very brief self-rated assessment of health at 40 which centred on a single question: "In general, would you say your health is excellent, very good, good, fair, or poor?" This cannot be expected to give a full indication of the woman’s physical and mental health status, disability or wellbeing – either at the current time or over previous years. Therefore, the overall analyses linking age at birth to "good" or "poor" health are quite open to interpretation and may be unreliable.
It is also possible that the links are being influenced by the confounding influence of other factors. The researchers have tried to take account of many things, such as proxy measures of socioeconomic status or longer-term health problems. However, they may not have been able to fully account for these and there may be various other personal or lifestyle characteristics having an influence.
A final point on applicability is that the study relates to a US population, where the different ethnic representation and other environmental factors mean that the results are not representative of other countries.
Ideally, a pregnancy should be planned, as you can take steps to increase your chances of a healthy pregnancy, such as quitting smoking if you smoke, achieving or maintaining a healthy weight, and taking folic acid supplements – read more about planning for a pregnancy.