Post-term births as a risk factor for small for gestational age births and infant mortality in Brazil, Mexico, and Palestinian refugees: An analysis of electronic birth records.

Background: Post-term pregnancy, defined as reaching or exceeding 42 + 0 weeks of gestation, is known to be associated with unfavourable birth outcomes. High-income countries have responded to this risk by widely adopting labour induction protocols in late-term, but many low- and middle-income countries have not. However, understanding underlying mechanisms linking post-term births to adverse newborn and infant outcomes remains limited.

Objective: To investigate the (a) prevalence of post-term, (b) the risk factors associated with post-term (c) the association between post-term births and the risk of small-for-gestational-age (SGA) neonates and of infant mortality in middle-income settings.

Methods: We used existing electronic datasets from the general population of Brazil, Mexico, and Palestinian refugees. Regression models were used to explore the associations between post-term birth and SGA and infant mortality.

Results: We analysed 21,335,033 live births in Brazil (2011-2018), 23,416,126 in Mexico (2008-2019), and 966,102 in Palestinian refugees (2010-2020) (N = 45,717,261). Post-term deliveries accounted for 3.1% of births in Brazil, 1.2% in Mexico, and 2.1% in Palestinian refugees. Post-term births had approximately three times the risk of resulting in SGA neonates compared to term births. Additionally, post-term neonates exhibited a 15% to 40% increased risk of infant mortality compared to term infants. Notably, post-term SGA neonates faced a significantly increased risk of infant mortality compared to term appropriate for gestational age neonates.

Conclusions: These findings emphasise the critical significance of implementing induction strategies to prevent post-term pregnancies and mitigate the associated risks of SGA neonates and subsequent infant mortality. Moreover, the study highlights the importance of accurately determining gestational age and using INTERGROWTH-21st charts to improve the identification of SGA cases, enabling targeted interventions. This is especially relevant because post-term SGA neonates may not exhibit low birthweight (a commonly used risk marker) and, therefore, may miss out on required specialised attention.