Data were available on a total of 34
845 women who had experienced a miscarriage in their first pregnancy and had a subsequent pregnancy. After the exclusion of multiple births and records with improbable interpregnancy intervals, 30
937 women constituted the study population. The figure presents the flow of population selection.
Flow of population selection
Of these 30
937 women, 12
744 (41.2%) conceived within six months of the miscarriage, 7791 (25.2%) after 6-12 months, 2958 (9.6%) after 12-18 months, 1995 (6.4%) after 18-24 months, and 5449 (17.6%) after 24 months.
Older women and those belonging to a higher social class tended to have shorter interpregnancy intervals (table 1). Compared with those with an interpregnancy interval of more than 24 months, women with an interpregnancy interval of less than six months were likely to be older (26.0 years (SD 5.4 years) v 23.9 (5.9) years; P<0.001) and from a less deprived Carstairs category at the time of their first pregnancy (4449 women, 34.9% v 1547 women, 28.4%; P<0.001), and less likely to have ever smoked (1261 women 9.9% v 963 women, 17.7%; P<0.001).
Table 1 Characteristics of women who had a second pregnancy after miscarriage, by interpregnancy intervals. Values are numbers (percentages) of women unless stated otherwise
Live birth rates were highest (10
856 women, 85.2%) in women with an interpregnancy interval of less than six months and lowest (n=3992, 73.3%) in those with an interpregnancy interval exceeding 24 months (P<0.01; table 2). The data on termination of pregnancies also displayed a similar pattern, with longer interpregnancy intervals associated with a higher chance of termination in the next pregnancy.
Table 2 Outcomes of second pregnancy after miscarriage in first pregnancy. Values are numbers (percentages)
After adjustment for maternal age at first pregnancy event, socioeconomic status, and year of first pregnancy, adverse pregnancy outcomes in women with an interpregnancy interval of less than six months differed significantly from the other groups (table 3). The models also included interaction terms: interpregnancy interval (six month bands) × year of first admission (five year bands). However, the interaction terms were found to be statistically significant only in the model for a second miscarriage (data available on request). Compared with the reference group (interpregnancy interval 6-12 months), women with an interpregnancy interval of less than six months were less likely to experience a miscarriage (adjusted odds ratio 0.66, 95% confidence interval 0.57 to 0.77), termination (0.43, 0.33 to 0.57), or ectopic pregnancy (0.48, 0.34 to 0.69). The highest risk of adverse outcomes was in women with an interpregnancy interval of more than 24 months, who were more likely in their second pregnancy to have an ectopic pregnancy (1.97, 1.42 to 2.72) or termination (2.40, 1.91 to 3.01).
Table 3 Crude and adjusted odds ratios of adverse pregnancy outcomes for different interpregnancy intervals
Including smoking status (when available) as a covariate in a subset of women showed similar patterns of risk, although some odds ratios (such as that for ectopic pregnancy or stillbirth) were no longer significant as the confidence intervals were widened to include 1 or the models did not converge because of smaller numbers. (Data available on request.)
Table 4 shows the proportions of women with maternal and perinatal complications in their second pregnancy. Women with an interpregnancy interval of less than six months were less likely to have caesarean section, preterm delivery (before 36 weeks), or an infant of low birth weight (<2500 g) but were more likely to have an induced labour (table 5). Women with an interpregnancy interval of more than 24 months were most likely to have a preterm delivery (1.21, 1.07 to 1.36) or very preterm delivery (1.40, 1.11 to1.70) than women with an interpregnancy interval of 6-12 months. They were also more likely to have a caesarean section (1.39, 1.26 to1.54) or an infant of low birth weight (1.23, 1.02 to 1.48).
Table 4 Maternal and perinatal complications by interpregnancy interval in women with live births in pregnancy after initial miscarriage. Values are numbers (percentages) of women
Table 5 Crude and adjusted odds ratios for complications in second pregnancy after initial miscarriage
No association was found between interpregnancy interval after miscarriage and pre-eclampsia, placenta praevia, or placental abruption in the second ongoing pregnancy.