Examining Interpregnancy Weight Change Across a Birthing Population in Aotearoa, New Zealand.

Introduction: Inter-pregnancy weight change (IPWC) has been linked to adverse outcomes in subsequent pregnancies. No studies have examined IPWC in Aotearoa New Zealand (AoNZ). This study aims to 1. Examine the distribution of IPWC in a birthing population in AoNZ and 2. Investigate IPWC across priority populations identified by the Perinatal and Maternal Mortality Review Committee (PMMRC).

Methods: This retrospective cohort study in AoNZ, included people who birthed their first two singleton pregnancies between 2016 and 2021. IPWC was calculated as the difference in pre-pregnancy body mass index (BMI) and divided into four categories: 1. Interpregnancy weight loss (IPWL) (BMI reduction of > 1 kg/m²) no change in weight (NC) (BMI change -1-0.9 kg/m²); moderate interpregnancy weight gain: (BMI increase 1-3 kg/m²) and increased IPWG (BMI increase > 3 kg/m²).

Results: The study cohort (n = 856) had a mean IPWC of 1.13 kg/m² (SD 3.5). 49.9% (n = 427) experienced a BMI increase (> 1 kg/m²). IPWC rates observed across categories: IPWL: 167 (19.5%); NC 262 (30.6%); IPWG: 427 (49.9%) (moderate IPWG: 235 [27.5%] and increased IPWG: 192 [22.4%]). IPWC varied by ethnicity, socioeconomic deprivation, age, and BMI in the index pregnancy; in a multiple regression model with these variables, increasing age and BMI in the index pregnancy were associated with lower IPWC.

Discussion: 22.4% of the cohort experienced IPWG at a level associated with adverse perinatal outcomes. Elevated rates of increased IPWG were observed in priority birthing populations, consistent with populations highlighted by the PMMRC. Further research is required to understand the impact of IPWC in AoNZ birthing populations.