Conceiving within a year of stillbirth is common and is not associated with increased risk of stillbirth, preterm birth, or small-for-gestational-age birth in the following pregnancy, compared with an interpregnancy interval of at least two years.

The results are from the first large-scale observational study to investigate the interval between stillbirth and subsequent pregnancy, including almost 14,500 births in women from Australia, Finland and Norway who had a stillbirth in their previous pregnancy. The findings are published in The Lancet.

The World Health Organization (WHO) recommends that women wait at least two years after a livebirth and at least 6 months after a miscarriage or induced abortion before conceiving again, but there is no guidance for the optimal interval after a stillbirth because there is limited evidence in this area.

"Our results consistently showed that an interpregnancy interval of less than one year was not associated with increased risk of adverse birth outcomes in the next pregnancy, compared with an interval of at least two years. Our findings provide valuable evidence for recommended pregnancy spacing after a stillbirth," says study author Dr Annette Regan, Curtin University, Australia. "Approximately 3.5 in every 1,000 births in high-income countries are stillborn, and there is limited guidance available for planning future pregnancies. We hope that our findings can provide reassurance to women who wish to become pregnant or unexpectedly become pregnant shortly after a stillbirth."

The study used birth records spanning 37 years (1980-2016) from Finland, Norway, and Australia to investigate intervals between pregnancies and the risk of subsequent stillbirth, preterm birth, and small-for-gestational-age birth. The authors note that these countries have access to universal health care and free antenatal care, and the populations are primarily white, so the findings might not be generalisable to low- or middle-income countries, countries without access to universal health care, or ethnic minority groups.

The study included singleton births only, and stillbirths following 22 or more weeks' gestation. The interpregnancy interval was calculated from the delivery date of the past birth or stillbirth and the start of the next pregnancy (delivery date of next pregnancy minus gestational age at birth), and was categorised as less than 6 months, 6-11 months, 12-23 months, 24-59 months, and more than 59 months.

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Overall, the study included 14,452 births among mothers who had a stillbirth in their previous pregnancy (4,170 in Finland, 6,761 in Norway, and 3,521 in Australia). Results were compared with 1,654,289 births following a previous livebirth from the three countries (536,392 in Finland, 854,999 in Norway, and 262,898 in Australia).

Of the 14,452 births in women whose previous pregnancy ended in stillbirth, 14,224 (98%) were livebirths, 2,532 (18%) were preterm births, and 1,284 (9%) were small-for-gestational-age births. Of the 228 stillbirths (2% of the total births), 201 (88%) were preterm and 27 (12%) were stillborn at term.

For women who had experienced stillbirth in their last pregnancy, intervals shorter than 12 months were not associated with increased risk of subsequent stillbirth, preterm birth, or small-for-gestational-age birth, compared with an interpregnancy interval of 24-59 months.

This trend remained the same when adjusted for maternal age, number of previous births, and decade of delivery. The authors also noted no difference in the association between interpregnancy interval and birth outcomes based on the gestational length of the previous stillbirth.

Short interpregnancy intervals were more common after stillbirth than after livebirth -- the median interpregnancy interval after a stillbirth was 9 months, compared with 25 months after a livebirth. After stillbirth, 9,109 (63%) women conceived their next child within 12 months, with 5,393 of those (37% of all births) conceived within 6 months.

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The authors note the difference in optimal intervals following livebirth and stillbirth. Dr Regan explains: "Although the mechanism linking interpregnancy interval and perinatal health is unclear, previous research offers several hypotheses, including depleted nutrition from past pregnancy, cervical insufficiency, and breastfeeding-pregnancy overlap in closely spaced pregnancies. Without sufficient time to recover from a previous pregnancy, women may be at increased risk of entering a reproductive cycle with poor nutritional status, which has been linked to increased risk of fetal growth restriction and birth defects. Such nutritional depletion might not occur to the same extent after a pregnancy loss, and this may affect the optimal interpregnancy interval, explaining why it may be different after stillbirth and livebirth."

The authors note that other factors that they could not study (such as maternal chronic medical conditions, pregnancy intention, use of assisted reproductive technology, cause of previous stillbirth, or socioeconomic status) may have affected their findings. They also add that women who conceive soon after a previous pregnancy might be healthier and more fertile than women who conceive later and therefore could be less prone to adverse birth outcomes.

Within the study, information on miscarriages or induced abortions was not available, which could have led to overestimation of interpregnancy interval in some women.

Lastly, the authors note that although this is largest study of its kind, only 228 women had recurrent stillbirths, which means the analyses for this group are limited due to small numbers. Replication of the study in a larger group would be informative.

Writing in a linked Comment, Mark A Klebanoff, The Research Institute at Nationwide Children's Hospital, USA, says: "The results of this study, in conjunction with results of studies of pregnancy interval after early loss and with findings of studies using new approaches to study interval after a livebirth, suggest that interpregnancy interval might be less important than previously assumed, at least in women in high-income regions. Rather than adhering to hard and fast rules, clinical recommendations should consider a woman's current health status, her current age in conjunction with her desires regarding child spacing and ultimate family size, and particularly following a loss, her emotional readiness to become pregnant again."