Is there now enough evidence to change how we manage the pregnancies of older mothers to reduce the risk of stillbirth?
I met with a 41-year-old woman this afternoon. Six weeks ago, she came to my hospital at 40 weeks and one day, concerned because she hadn’t felt her baby move for some hours. On arrival to hospital no fetal heartbeat could be heard and the ultrasound scan confirmed the baby had died. We talked today for some time about the list of stillbirth investigations that had been undertaken, in accordance with our hospital protocol (derived from the Perinatal Society of Australia and New Zealand Clinical Practice Guidelines) and the rationale behind these tests. They were all normal. She asked me if anything could have been done to prevent this. If I was asked this question five years ago I would have said I don’t know. I still told her that I don’t know, but I think there is now a little evidence and some guidance around timing of birth for older women.
More than one in five women giving birth in Australia are aged older than 35 years. As women age, fertility declines and pregnancies are associated with higher rates of antenatal and intrapartum maternal complications. Stillbirth is also more common as women get older. When the risk of stillbirth is expressed as a proportion of ongoing pregnancies at a given gestation, it is evident that the risk is greatest among term and post-term pregnancies.1 In very large retrospective cohorts, the risk of stillbirth is twice as common in women aged older than 35 years2, and three times as common in women aged 40–44 years3 compared to women aged 25–29. Recent Australian data4 published from my own hospital showed that women aged 40 years or more accounted for 3.5 per cent of all births yet 8.5 per cent of term stillbirths. Even when adjusted for modifiable risk factors, such as obesity and smoking, maternal age remains an independent risk factor for stillbirth.
It is not entirely clear why stillbirth is more common as women get older. Older women are more likely to be overweight and associations have been demonstrated between increased BMI and stillbirth. There are data showing that fetal growth restriction (FGR) increases with maternal age and there are also proven associations between small for gestational age (SGA)/FGR and stillbirth. However, the rate of FGR in the stillborn babies of older mothers is not greater than in younger mothers.5 Presently, there is no evidence to support routine ultrasound scanning for fetal growth or umbilical artery and uterine artery Dopplers in older women.
Induction of labour at term presents as a potential strategy to reduce the risk of stillbirth for older women. More generally, there is Level 1 evidence that induction of labour for post-term pregnancies reduces perinatal mortality without increasing the caesarean section rate. Contrary to many clinicians’ opinions, there is also data showing improved perinatal outcomes and no difference in caesarean section when induction of labour is routinely performed prior to term. In the Hypitat trial, for example, induction of labour after 37 weeks for pregnancies complicated by a hypertensive disorder was associated with no difference in mode of birth compared to those women randomised to expectant management. Inferences can therefore be drawn from studies of induction of labour among women of all ages, that induction of labour in older women at, or prior to, term may hold the promise of preventing some stillbirths, without increasing the woman’s chance of birth by caesarean section.
Earlier this year, the Royal College of Obstetricians and Gynaecologists published ‘Induction of Labour at Term in Older Mothers’.6 In this paper they modelled that if all UK women aged 40 years or older with a singleton pregnancy were induced at 39 weeks instead of 41 weeks, 17 stillbirths could be prevented. This equates to inducing an extra 550 women to prevent one stillbirth. Inducing at 40 weeks instead of 41, would prevent seven stillbirths and require an extra 679 inductions to prevent one stillbirth. And, in a similar analysis, Fretts7 hypothesised that among women aged 35 years or older, a more modest figure of 71 additional inductions at 39 weeks would be required to prevent one unexplained stillbirth. The underlying reason to recommend induction of labour in the setting of a post-term pregnancy is to prevent stillbirth. While the number needed to treat may appear large, it is noteworthy that the absolute risk of stillbirth at 38 weeks in women aged 40 years or older is still greater than the absolute risk of stillbirth at 41 weeks in younger women.8 There is currently no prospective study specifically assessing the maternal and neonatal outcomes of older women undergoing induction of labour at, or prior to, term compared to expectant management. There is a need for data to demonstrate this benefit and to also quantify the likelihood of additional obstetric intervention (if any) and its associated cost.
The risk of stillbirth in high-income countries has shown little or no improvement in the past 20 years. The recent Lancet stillbirth series9 has articulated the need for resources to prioritise stillbirth research and prevention strategies. Maternal overweight and obesity, and smoking are the most important potentially modifiable risk factors for stillbirth. It is not possible for any of us to become younger, yet the stillbirth risk associated with maternal age is able to be somewhat addressed through increased awareness and careful consideration as to the most appropriate time to have children. Based on the available data, induction of labour at or near term for older mothers is likely to be associated with a reduction in stillbirth risk without increasing the likelihood of birth by caesarean section.