Table 2.

Summary of evidence

Outcome	Studies (k) Study designs Observations (n)	Summary of findings	Consistency and precision	Other limitations	Strength of evidence	Applicability
Preterm birth	14 studies (cohort); N = 2 557 668	Risk was significantly higher with shorter IPI in 10 studies (aOR ≥ 1.20 for ~<6 mo in 10 studies, but only four of six good‐quality studies; point estimates decreased with increasing IPI).	Inconsistent in good quality studies, precise	Limited adjustment for confounders and validity of US vital statistics‐based data sources.	Moderate	High
Spontaneous preterm birth	2 studies (cohort); N = 176 177	Risk was significantly higher with shorter IPI in 1 study (aOR = 1.83 for <6 mo; aOR = 1.26 for 6‐11).	Inconsistent, imprecise	Few studies; limited adjustment for confounders and validity of US vital statistics‐based data sources.	Low	Moderate
Small‐for‐gestational age	11 studies (cohort); N = 1 184 143	Risk was significantly higher with shorter IPI in 5 studies (aOR ≥ 1.20 for <6 mo in five studies, but none were good‐quality studies).	Inconsistent, precise	Limited adjustment for confounders and validity of US vital statistics‐based data sources.	Low	High
Perinatal death	4 studies (cohort); N = 610 829	There were non‐significant increased risks with shorter IPI in two studies for <6 mo.	Inconsistent, imprecise	Few studies; variation in outcome definition, limited adjustment for confounders and validity of US vital statistics‐based data sources.	Low	Moderate
Infant mortality	4 studies (3 cohort and 1 case‐control); N = 220 676	Risk was significantly higher with shorter IPI in 4 studies (aOR ≥ 1.20 for <6 mo in 4 studies; 6‐11 mo in 1 study; 12‐17 mo in 1 study).	Consistent, precise	Few studies; variation in outcome definition, limited adjustment for confounders, and validity of US vital statistics‐based data sources.	Moderate	Moderate

IPI, interpregnancy interval; aOR, adjusted odds ratio.