Growth patterns in preterm infants in Iran (the PERSIAN Birth Cohort): a prospective birth cohort study

Childhood growth impairment, often due to undernutrition, increases long-term health risks. Preterm infants are especially susceptible and require early support. Catch-up growth helps, but its optimal timing is unclear. This study examined preterm growth patterns to identify those at risk and guide intervention.

Using PERSIAN Birth Cohort data from four Iranian cities, we examined how preterm birth affects height-for-age Z score (HAZ), weight-for-age Z score (WAZ), weight-for-height Z score (WHZ), BMI-for-age Z score (BMI-Z), stunting, wasting, underweight, and obesity. Preterm status was categorised into full term (≥37 weeks), late preterm (34 to <37 weeks), moderate preterm (32 to <34 weeks), and very or extremely preterm (<32 weeks). Growth effects were assessed via generalised estimating equations. Quantile regression examined preterm birth's effect across BMI-for-age Z score quantiles. This method enables assessment of whether the impact of preterm birth differs across the BMI-Z distribution, offering insights beyond average effects.

Among 9015 pregnant Iranian women enrolled, 7169 neonates were followed up from ages 2 months to 60 months. 10·48% of births were preterm (8·75% late preterm, 1·26% moderate preterm, and 0·52% very or extremely preterm). By 36 months, both late and moderate preterm children showed catch-up in HAZ, and by 48 months, in WAZ, with no significant differences in stunting or underweight risk compared with full term peers. In contrast, very or extremely preterm children had persistent HAZ and WAZ deficits throughout the follow-up period and remained at higher risk of underweight, although stunting risk was no longer elevated after age 36 months. WHZ and BMI-Z declined with increasing prematurity and remained lower up to age 60 months. All categories of preterm birth were associated with higher odds of wasting and lower BMI-Z from ages 2 months to 60 months, with no significant difference in obesity risk.

Preterm infants face persistent growth challenges, including an increased risk of wasting, despite similar height-for-age and weight-for-age to full term peers. These findings highlight the need for early, targeted nutritional interventions to prevent wasting and support optimal growth.

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