The Interpregnancy Interval Revisited

Abstract & Commentary

Synopsis: Interpregnancy intervals less than six months or greater than 120 months are associated with increased risks of low birth weight, preterm birth, and neonates who are small for gestational age.

Source: Zhu BP, et al. N Engl J Med 1999;340: 589-594.

Relatively little is known about how the interpregnancy interval affects perinatal outcomes. Zhu and colleagues examined vital statistics data in Utah, where the average parity is high and confounding risk factors, such as substance abuse, are less prevalent. Zhu et al looked at 173,205 live births between 1989 and 1996 to evaluate three adverse outcomes: low birth weight, preterm birth, and small size for gestational age (SGA), comparing these with the interpregnancy level. They attempted to control for numerous potential cofounders: maternal age, height, weight, number of prenatal visits, race, education, substance abuse, and other factors.

As expected, risks of low birth weight, preterm birth, and SGA birth were high when the interpregnancy interval was less than three months. With intervals less than six months, there was a 40% increased risk of low birth weight (95% CI, 1.3-1.6), a 40% increased risk of preterm birth (95% CI, 1.3-1.5), and a 30% increased risk of SGA delivery (95% CI, 1.2-1.4). The risks dropped off rapidly with an increasing interpregnancy interval. The lowest risks correlated with an interval of 1.5-2 years.

When the interpregnancy interval exceeded two years, risks slowly began to rise again. After a 10-year interval, there was a doubled risk of low birth weight (95% CI, 1.7-2.4), a 50% increase in preterm birth (95% CI, 1.3-1.7), and an 80% increase in SGA births (95% CI, 1.6-2.0). Both short and long interpregnancy intervals persisted as independent perinatal risk factors after controlling for confounders.

Comment by Elizabeth Morrison, MD, MSEd

With periconception care prominently featured in the draft objectives of the Health People 2010 initiative,1 researchers are paying more attention to birth spacing and related issues. Few recent studies have shed light on the relationship between the interpregnancy interval and pregnancy outcomes. This study provides intriguing information that can help us counsel parents as they plan to expand their families.

As we would expect, short interpregnancy intervals correlate with adverse perinatal outcomes, though not any more strongly than primiparity does. This relationship has been attributed to a postpartum "maternal depletion syndrome." The more surprising finding was that longer interpregnancy intervals are associated with adverse outcomes, which, in some cases, may exceed the risks of short intervals. It is not clear what causes this relationship. Zhu et al hypothesize that beneficial maternal physiologic characteristics may gradually revert back to the primigravid state after a previous gestation, or that unmeasured confounders may lead both to long interpregnancy levels and to adverse birth outcomes. A limitation in the study is its fairly low representation of nonwhite mothers, whose interpregnancy intervals may affect outcomes differently.

In any case, the discovery by Zhu et al of a J-shaped association between interpregnancy interval and perinatal outcomes provides useful information for clinicians providing periconception care. The American College of Obstetricians and Gynocologists (ACOG) recommends that we discuss birth spacing when we provide preconception counseling.2 As we discuss parents’ desires for family planning, we can now offer families more conclusive information about the optimal interpregnancy interval.


    1. Washington, 1999.

    2. Commitee on Technical Bulletins, ACOG, 1995.