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"Older" motherhood

The rise in maternal age is part of an ongoing social trend over the past five decades. It reflects the fact that in the twenty-first century, women have access to effective contraception, and this gives them the scope to construct their lives around choices about partners, careers, and friendships.

There is no right to have a child, and it is not always possible to control whether one becomes pregnant or not. But in twenty-first-century Britain, it is not generally biology that pushes women to have babies, but personal decision-making, which takes place within a wider social context.

Some of the social and cultural trends that affect the timing of motherhood are worthy of debate. That women are having babies later in life is neither an obviously good thing, or an obviously bad thing. But many of these social and cultural debates tend to be played out through a discussion of the clinical problems about older women's ability to carry a pregnancy in their thirties and give birth to a healthy child.

This tends to simplify and distort the science around fertility, and panic many women when their chances of having a baby at the time that is right for them are high. Research by BPAS shows there is disproportionate concern among women about their fertility, and a tendency to overestimate the difficulties that may be encountered conceiving at the age of 35.

Research indicates that 'within a year, 75% of women aged 30 and 66% of women aged 35 will conceive naturally and have a baby. After this, it is increasingly difficult to fall pregnant, and the chance of miscarriage rises.' Age-related infertility is a continuum, and it does indeed become 'increasingly difficult to get pregnant' over the age of 35. But the question is precisely how much more difficult it becomes.

The widely citied statistic that only 66% of women aged 35 to 39 will be pregnant after a year of trying if based on a 2004 article in the journal Human Reproduction, which in turn is based on an analysis of French birth records from 1670 to 1830. The attraction of using data from a pre-modern population is the fact that the data is not distorted by the use of birth control. But as the psychologist Jean Twenge points out in her review of the literature, this was "a time before electricity, antibiotics, or fertility treatment".

For modern women, the reality is far better expressed in a 2004 study by David Dunson and colleagues, published in Obstetrics and Gynecology. This found that, if they were having sex twice a week, 82% of women aged between 35 and 39 fell pregnant within a year. However, what made the biggest difference was the frequency of intercourse: if women had sex once per week instead of twice, 'the rates of infertility increase substantially to 15%, 22–24%, and 29% for women aged 19–26, 27–34, and 35–39 years, respectively'.

Dunson et al. also found that 'age of the man also has a large effect on time to pregnancy and the proportion of couples classified as clinically infertile'. 'For men younger than 35 years, there is no effect, but starting in the late 30s, the impact of male age becomes pronounced,' they write. 'The effect on fertility of a man aging from 35 to 40 is about the same as the effect seen when intercourse frequency drops from twice per week to once per week'.

Dunson et al. concluded that:

'Increased infertility in older couples is attributable primarily to declines in fertility rates rather than to absolute sterility. Many infertile couples will conceive if they try for an additional year'.

In other words: women in their thirties might have to try harder to get pregnant, particularly if their partner is also in his late thirties. However, their age alone is unlikely to make them infertile.

The science of the issue is backed up by our experience of modern life. The social trend towards later motherhood, which is evidenced by the growing number of mothers in their 30s, indicate that getting pregnant is not a problem for the majority of women aged 35-39.

However, this does not mean that all women who want to get pregnant in their late 30s will be able to – and this is really where the problem lies. Population-level data cannot predict individual experience; so statistics cannot tell us when, exactly, an individual woman's fertility begins to decline. We know that in general, women aged 35-39 have a reasonable chance of getting pregnant; but when an individual woman aged 36, or 38, tries to become pregnant she might not always succeed.

This could be related to her age: for example, the RCOG notes that early ovarian ageing happens in around 10% of women in the general population. In other words, a woman in her late 30s may experience infertility not because she is 'typical' of women her age, but because she is untypical. At a purely biological level, if this woman had tried to conceive earlier, she would have had a better chance of becoming pregnant. However, even these situations require caveats.

Like early menopause and a host of other problems that can lead to infertility, or difficulties in conceiving, such as endometriosis, polycystic ovary syndrome, or blocked fallopian tubes, early ovarian ageing can happen at any point during a woman's reproductive lifetime; and unless a woman is actively trying to become pregnant, she may not realise that she has these problems.

It is misleading to call these problems of 'age-related infertility' – they are just problems of fertility. Where it relates to a woman's age is in the implications of resolving the problem. And it is this, in fact, where much of the recent angst about the 'biological clock' comes from.

While the risks of pregnancy complications increase with age, this does not mean that women having babies at the age of 35 or over are de facto putting themselves or their babies at increased risk. Most pregnancies will result in a healthy baby. However, adverse pregnancy outcomes also rise with age, and women over 40 are considered to be at a higher risk of pregnancy complications.

A host of factors affect a woman's likelihood of developing pregnancy complications or adverse neonatal outcomes, from her socioeconomic status to her general health; and maternal age is best understood as one of these factors, rather than the sole determinant.

As a 2014 BJOG commentary by WR Cohen spells out, women are all different, and 'chronological age does not necessarily equate with an individual's biological age or health'. If there were 'some biomarker of general ageing, and of reproductive ageing in particular', this would enable us to 'identify the large subset of women over 35 or 40 whose pregnancy-related risks are not substantially increased by their age, and others whose probable outcomes engender less optimism.'

Whatever the age of the mother, the process of pregnancy and birth themselves pose a level of risk to the woman and her baby, through hypertension, bleeding, difficult labours and so on. The issue at stake here is only how much additional risk is posed by advanced maternal age.

One of the most well-known risks of later maternal age is the incidence of certain types of fetal anomaly, in particular Down's, Edwards' and Patau syndrome – or trisomies 21, 18 and 13. Over half of babies with Down's syndrome are born to mothers who are under 35. However, the overall risk for women having a baby at the age of 40 remains relatively low – 99 out of 100 women will not have a pregnancy affected by Down's syndrome. With Down's syndrome, the age breakdown is:

  • 25 years of age has a risk of 1 in 1,250
  • 30 years of age has a risk of 1 in 1,000
  • 35 years of age has a risk of 1 in 400
  • 40 years of age has a risk of 1 in 100
  • 45 years of age has a risk of 1 in 30

Even if they have not undergone fertility treatment, older women are also naturally more likely to have twins or triplets, which as well as having a higher risk of congenital anomalies (around 5% more common in multiple pregnancies than in singleton pregnancies), are also at risk of growth restriction and preterm birth, which in turn is associated with other complications such as cerebral palsy and learning difficulties. Multiple pregnancy is also associated with higher risks for the mother. Women carrying more than one baby have an increased risk of anaemia, hypertensive disorders, haemorrhage and postnatal illness. In general, maternal mortality associated with multiple births is 2.5 times that for singleton births.

The clinical objections to delayed motherhood can be summed up in two statements. In terms of one's reproductive capacity, it is better to have babies younger; and in terms of one's health generally, it is better to be younger. It is impossible to argue convincingly against either of these statements in their own terms. They are just true.

With all this going on, doctors and policymakers need to tread very carefully when issuing messages to women about how they time their childbearing. On one hand, it is important to be honest, and recognise that a woman who is biologically infertile cannot get pregnant just because she wants to. At a general level, the evidence strongly suggests that women are likely to find the process of conceiving, being pregnant, and giving birth more straightforward if they are under 40, and there is little to be gained from denying that this is the case.

We should also be honest in saying that the context in which women make their fertility choices is not necessarily ideal, and it would be better if young mothers stood a decent chance of being career women too. Questioning the demands placed on women by today's culture of 'intensive parenting', and putting the case for affordable, flexible childcare, are as important for women as is protecting their ability to plan the timing of their families through contraception and abortion.

But part of being honest means that we should not over-inflate the problem. The continual conflation of the problems facing women at aged 35 with those aged over 40 seems deliberately designed to present women with a worst-case scenario, as though there is a need to scare women in their early thirties into rushing into pregnancy. The likely reality is that more women will have children in their mid to late thirties, and the likely outcome of that is that most pregnancies, births, and babies will be healthy.

Fertility treatment is expensive and uncertain, but most women know that already: and they also know that it can work in some cases. Being aware that fertility treatment exists as a last resort does not encourage women to 'put off' having babies, just as knowing that abortion is there when contraception fails does not stop women from using contraception. Women respond, not to journal articles and pronouncements from Royal Colleges, but to their personal circumstances and the experiences of those around them. These circumstances and experiences suggest to them that having babies in one's thirties is quite normal.

If policymakers are worried by the consequences of later maternal age, they should be focusing on supporting prenatal screening services, preparing maternity services to be better able to cope with obstetric complications, and funding decent miscarriage care and fertility treatment. What they should not be doing is nagging women to get pregnant before they are ready, just so they fit neatly into the 'period of optimum fertility'.