Individualised risks of stillbirth at advanced maternal age: A literature review of the evidence

A Corresponding Author: morgan.e. weathington @gmail.com B Self-employed LMC Midwife, Wanaka C Otago Polytechnic School of Midwifery INTRODUCTION There is a growing trend in developed nations such as the United Kingdom (UK), Western European countries, Australia, Canada and New Zealand (NZ) for women to delay childbearing to later in their lives (Royal College of Obstetricians and Gynaecologists [RCOG], 2009). From 2005 to 2014 the birth rate for NZ women over 40 increased by 17%, from 13 to 15.3 births/1000 women of reproductive age (Ministry of Health [MOH], 2015a). The reasons women are making this choice are often associated with the availability of safe effective contraception, continuing education, career development, achieving financial independence, seeking a stable relationship with a supportive partner, sophisticated fertility treatments, and shifts in the female gender role (Carolan, 2003; RCOG, 2009). In the last 100 years, the notion that “older” mothers are at higher risk in pregnancy and birth has flourished (Hallgrimsdottir & Benner, 2014). There is a large volume of literature, although inconsistent, associating increasing risk with advancing maternal age (Carolan & Frankowska, 2011; Flenady et al., 2011; Kenny et al., 2013). AMA pregnancies (defined in this paper as mothers >40 years) are associated with higher rates of hypertension and diabetes (Cleary-Goldman et al., 2005; Joseph et al., 2005), and women in general will accrue skeletal damage, cancers and other underlying medical conditions as they age (Suplee, Dawley, & Bloch, 2007). AMA is also associated with a range of adverse pregnancy outcomes: low birth weight, preterm birth, stillbirth and unexplained death, and increased rates of assisted births and caesarean sections (Bayrampour & Heaman, 2010; Cleary-Goldman et al., 2005; Flenady et al., 2011; Fretts, 2005; Herstad et al., 2014; Tough et al., 2006). However, there is lack of consensus in the literature regarding at what age this risk becomes significant (Carolan & Fankowska, 2011). Further, the underlying cause of these pregnancy outcomes, when the known pathophysiology of underlying disease is not the cause, is yet to be determined. Currently in NZ, it is common to recommend induction of labour (IOL) at 39 weeks gestation for women of AMA beyond age 40 (Canterbury District Health Board [CDHB], 2014; Capital & Coast District Health Board [CCDHB], 2015; Wise et al., 2014). Over 40% of women over the age of 40 are being induced (MOH, 2015b), although the reason for these inductions is unclear. A guideline of IOL for AMA after 39 weeks is seen as a way to minimise the risk of AMA and term complications, mainly stillbirth. However, the routine or blanket application of a guideline fails to account for individualised risk assessment as happens within the NZ model of midwifery partnership. Population-based studies (national datasets), although powered to detect a rare event such as stillbirth, are consistent in reporting an association with women aged over 40 and stillbirth (1.72-3.3 OR, Introduction: Women worldwide are having their babies later than in previous generations. Advanced maternal age (AMA) has been associated with adverse pregnancy outcomes, the most severe being increased risk of stillbirth. This literature review examines the evidence for risk of stillbirth at term for women aged 40 or over in high income countries, and outlines factors found in the literature that amplify or mitigate the risk. The aim is to assist New Zealand (NZ) AMA women to make informed choices around induction of labour (IOL) which is frequently recommended at 39 weeks gestation as a general district health board (DHB) guideline for women of AMA. Methods: Databases searched included Google Scholar, CINAHL, Science Direct, PubMed, ProQuest, and Cochrane. Seven papers met the inclusion criteria. Stillbirth risk was assessed against the exacerbating or mitigating factors within the individual papers and compared, if appropriate. Guideline documents regarding NZ obstetric recommendations for AMA were also hand searched. Findings: The risk of stillbirth is associated with increasing maternal age beyond age 40 with the range of adjusted odds ratios (AORs) of 1.43 to 3.04. However, absolute risk of stillbirth for Australian and NZ women over age 40 remains very low at 4.05 stillbirths/1000 undelivered pregnancies, or an absolute risk factor above a baseline rate of 3.4/1000 for stillbirths (>24wks). Further, the stillbirth risk for women of AMA may be modified by parity and general health. Conclusion: AMA is associated with increasing stillbirth rates, although the absolute rate remains low and can potentially be modified by parity and general health. Therefore, women of AMA in the NZ context have the opportunity to assess their overall risk in consultation with their maternity carer, including these factors, when making a decision about early term IOL.