A Rise in Uncertainty: Ill-Defined and Perinatal Infant Deaths in New Zealand

A Rise in Uncertainty: Ill-Defined and Perinatal Infant Deaths in New Zealand

Overview

This article examines a striking rise in infant mortality classifications under ICD Chapter 18 (Ill-defined conditions) and Chapter 16 (Certain conditions originating in the perinatal period) from 2020 to 2022, based on official Ministry of Health data. The increase is particularly evident in infants under age 1, and in the case of ill-defined causes, the pattern appears almost exclusively limited to this group.

Key Findings – Chapter 18: Ill-defined Causes

• The subgroup R95–R99: Unknown and unspecified causes of mortality saw a dramatic rise in infants (age 0) starting in 2020.
• This increase remained elevated through 2021 and 2022, despite historically stable numbers from 2013–2019.
• A comparison with the broader age 00–04 cohort showed a nearly identical trend, confirming that:
  

  The entire rise in Chapter 18 deaths in young children is being driven by infants.

• Other ill-defined subgroups, such as R00–R09 (respiratory symptoms) or R40–R94 (other signs) remained flat.

This points to a specific post-2020 increase in cause-of-death uncertainty or non-attribution in neonatal mortality certifications.

Chapter 16 – Perinatal Conditions

Infant deaths attributed to perinatal causes also rose post-2020, particularly in:

• P90–P96: Other disorders originating in the perinatal period also increased steadily over the same timeframe.

A focused view of the top 5 ICD subgroups in Chapter 16 from 2013–2022 reveals:

1. P05–P08 (Gestation length/fetal growth issues) spiked sharply in 2014 and again post-2020.
2. P20–P29 (Neonatal respiratory/cardiovascular disorders) remained steady with a mild increase.
3. P90–P96 (Other perinatal disorders) rose significantly after 2020.
4. P50–P61 (Haemorrhagic/haematologic) showed sustained moderate increase.
5. P35–P39 (Perinatal infections) began rising post-2020.

These changes, particularly the increase in disorders linked to maternal health or intrauterine factors, suggest either:

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• A real shift in neonatal health outcomes during the pandemic and vaccine rollout era,
• Or a change in diagnostic and attribution practices.

Notably, the 1–4 age group does not show similar trends, reinforcing the interpretation that these mortality shifts are concentrated in infants only.

Data Source: Ministry of Health NZ, mortality_Chapt_Subgrp.txt (2013–2022).

Were More Babies Born?

To test whether these increases might simply reflect a growing infant population, we examined annual average birth cohort sizes (proxied by age 0 population):

The 2021 birth cohort was modestly larger than surrounding years, but not enough to explain the sharp post-2020 increases in infant deaths. 2020 and 2022 were typical and align with pre-pandemic norms.