Factors associated with having the first baby at an advanced age

Abstract

The aims of this thesis were to investigate characteristics of women and men who have their first baby at an advanced age, reasons for postponing childbirth, and consequences in terms of adverse pregnancy outcomes.

Maternal age at first birth has increased in many modern societies; in both Sweden and Norway, first-time mothers are now about five years older, compared with the previous generation. This delaying of parenthood has been associated with an increased need for artificial reproductive techniques and adverse pregnancy outcomes, and it may also contribute to reduced fertility rates in a society.

In Studies I and II, data from the Norwegian Mother and Child Cohort Study were used, and the characteristics of 41 236 women and 14 832 men who had their first baby during the period 1999-2008 were investigated. Compared with younger reference groups (women aged 25-32 years; men aged 25-34 years), maternal age ≥33 years and paternal age ≥35 years were associated with fecundity problems and slightly more health problems and risky health behaviour. The vast majority of older first-time parents had a high level of education and annual income. However, a smaller group was socio-economically more disadvantaged with low level of education, single status, unemployment, unsatisfactory relationship with their partner and had an unplanned pregnancy.

Studies III and IV were based on data from the Swedish Young Adult Panel Study and the Swedish Total Population Register. Reproductive intentions and reasons for not having children at the age of 28, 32 and 36 or 40 were investigated in 365 childless women and 356 childless men. Many overestimated their fecundity, and one in three women and men aged 36 or 40 years wished to have children. Reasons for remaining childless at these ages were: lack of partner, no desire for children, not feeling mature enough, and wanting to explore other aspects of life before having a family (III). Predictors of still being childless at the age of 32 were investigated in 22-year-olds (518 women and 482 men). These predictors were: family background factors such as growing up in a large city, having highly educated parents, being an only child, still living in their original family, having a less than positive experience of their own mother and father as parents, an unsatisfactory relationship with their mother, and negative attitudes to children (IV).

Study V was a population-based register study including 955 804 primiparous women from the Swedish and Norwegian Medical Birth Registers who gave birth during the period 1990-2010. The risk of preterm birth, infant small for gestation age, low Apgar score, stillbirth and neonatal death was investigated in women aged 30-34 years, 35-39 years and ≥40 years compared with women 25-29 years of age. Additionally, the risks associated with advanced maternal age were compared with those of smoking and being overweight or obese. The adjusted Odds Ratios (aOR) of all outcomes increased by maternal age in a similar way in Sweden and Norway, and there was a risk of fetal death already at the age of 30-34 years (Sweden aOR 1.24; 95% Confidence Interval (CI) 1.13–1.37, Norway aOR 1.26; 95% CI 1.12–1.41). The Swedish data showed that maternal age ≥30 years was a risk associated with the same number of additional cases of fetal deaths (n=251) as overweight/obesity (n=251).

In conclusion, this thesis confirms some of the findings from previous research regarding the selection of women who delay childbirth to advanced age, such as well- educated women and high-income earners. It adds information about the characteristics of first-time fathers and it also shows that a minority of first-time parents constitute a less advantaged group. The prospective longitudinal study suggests that, besides well- known factors, young persons’ experience of their own parents may have an impact on reproductive behaviour, especially the relationship with the mother. Finally, the thesis confirms the association between advanced maternal age and severe pregnancy outcomes, but adds to previous knowledge that risk may increase already at the age of 30-34 years.