Fourteen observational studies were included: 11 retrospective cross-sectional studies and three prospective cohort studies. Sample sizes ranged from 309 to 50,596 women. All studies had a weakness in their study design and analysis and some were described as having fundamental flaws. There was a risk of recall bias in the retrospective studies. Four studies did not adjust for known confounders. Five studies did not control for breast-feeding. Six studies did not control for sterilisation.
The results of four studies that used univariate analyses that did not control for possible confounding factors suggested a trend towards longer birth intervals for contraceptive users compared with non users. Without adjustment for confounding factors assessment of the strength of the association was not possible.
The results of five studies that used multivariate analyses and a predefined short birth interval (range five to 24 months) were mixed. One study of women in Pakistan found that contraceptive users had a shorter interval between births compared to non users. The other studies suggested a protective effect against short birth intervals, but the strength of the relationship was variable. The protective effect ranged from a modest increased odds of a short birth interval (OR 1.23, 95% CI 1.10 to 1.38) with injectable contraceptives compared to no or other methods based on data from four sub-Saharan countries to a strong protective effect of insertion of implants in a US study; the odds of conception in the first year following a birth were greatly increased for users of other or no method of contraception (OR 35.2, 95% CI 4.48 to 276.4). One study found that although contraceptive use protected against birth intervals of 15 months or less, the protective effect was reversed by concurrent breast feeding.
The results of five studies that used multivariate models and a continuous measure of the time interval between births were mixed. One study of Nigerian women found that contraceptive users had shorter birth intervals by an average of two weeks compared with non users. The other studies reported longer birth intervals for contraceptive users (range 1.72 months to 20 months, where reported).