Interpregnancy Intervals: How Long is Long Enough Between Pregnancies?
Abstract & Commentary
Synopsis: Although the incidences of adverse birth outcomes in general were higher in African-American women, the odds ratios for each adverse outcome at various pregnancy intervals were remarkably similar.
Source: Zhu BP, et al. Am J Obstet Gynecol. 2001; 185(6):1403-1410.
Zhu and colleagues examined a michigan state database in order to determine which interpregnancy interval was associated with the best outcome. Their data set involved live births between 1993 and 1998. Pregnancy interval was defined as the difference in months between 2 births minus the length of gestation of the second pregnancy. The 3 outcome variables they evaluated were the incidences of preterm birth (< 37 wks), low birth weight (< 2500 g), and small-for-gestational age (< 10th percentile of mean weight for gestation). Also, since this had not been evaluated before, Zhu et al separated data from African-American and Caucasian mothers, to see if the ideal interpregnancy interval differed between races.
They found that, although the incidences of adverse outcomes in general were higher in African American women, the odds ratios for each adverse outcome at various pregnancy intervals were remarkably similar. For example, the ideal pregnancy interval for the entire population was between 18 and 23 months. If the interval was less than 6 months, the odds ratio of any of the above adverse outcomes was between 1.3 and 1.5, and if one waited for more than 120 months to become pregnant, the chances of adverse outcome were increased by 1.3-1.9 fold.
Comment by John C. Hobbins, MD
One of the most common questions asked at the time of discharge and/or at the 6-week postpartum visit is "when can I become pregnant again?" Realizing that a very short interval between pregnancies was not a good idea, but, sadly, not having researched this area well enough, I have cavalierly responded "you should wait at least 6 months before trying to become pregnant." Based on this well-conducted study, I was right to a certain extent, but I missed the optimal gestational interval of 18-23 weeks.
There are a number of reasons why a short interval could be associated with a higher rate of adverse outcome. These would include the possibly simplistic idea of failure of the uterus to fully return to its normal "receptive" state, inadequate return to optimal nutritional balance, and the stress of taking care of a young infant. However, there are obviously confounding valuables such as smoking and unplanned pregnancy, which can affect outcome.
The results regarding a prolonged interval should not be surprising. Incipient diabetes and hypertension would have a greater chance of surfacing in a patient who is 10 years older. Also, secondary infertility may predispose patients to a higher rate of problems once they become pregnant, and "older eggs" may not necessarily be primed for an ideal pregnancy outcome.
In any case, I learned something from this article and its precursors that will allow me to better counsel patients—this time with a little more conviction.
Dr. Hobbins is Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver.
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