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Australia; WA
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New Curtin University-led research has called into question existing health advice that mothers wait a minimum of two years after giving birth to become pregnant again, in order to reduce the risk of adverse pregnancy outcomes, such as preterm and small-for-gestational age births.
The research found that a World Health Organization (WHO) recommendation to wait at least 24 months to conceive after a previous birth may be unnecessarily long for mothers in high-income countries such as Australia, Finland, Norway and the United States.
Lead researcher Dr Gizachew Tessema from the Curtin School of Population Health said because the WHO advice was based on limited evidence from resource-limited countries, it was necessary to investigate whether the 15-year-old recommendation was relevant for higher-income settings.
“We compared approximately 3 million births from 1.2 million women with at least three children and discovered the risk of adverse birth outcomes after an interpregnancy interval of less than six months was no greater than for those born after an 18-23 month interval,” Dr Tessema said.
“Given that the current recommendations on birth spacing is for a waiting time of at least 18 months to two years after livebirths, our findings are reassuring for families who conceive sooner than this.
“However, we found siblings born after a greater than 60-month interval had an increased risk of adverse birth outcomes.”
Dr Tessema said just as the current WHO recommendations are not age specific, the study’s results were not necessarily equally applicable to parents of all ages.
“Our next step with this research is to identify whether intervals between pregnancies affect the risk of adverse birth outcomes among women of different ages,” Dr Tessema said.
Dr Tessema is a perinatal and reproductive epidemiologist and conducted the study with senior author Professor Gavin Pereira, who are both from the Curtin School of Population Health and the new Curtin enAble Institute.
The international study was funded by the National Health and Medical Research Council, UK Medical Research Council, Research Council of Norway and National Institutes of Health.
The paper, ‘Interpregnancy intervals and adverse birth outcomes in high-income countries: an international cohort study’, was published in journal PLOS ONE and can be found online here.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Associate Professor Alex Polyakov is a Clinical Associate Professor in the Faculty of Medicine, Dentistry and Health Sciences at the University of Melbourne and is a Medical Director at Genea Fertility Melbourne
Inter-pregnancy interval
World health Organisation recommends to waiting at least 24 months before attempting pregnancy following a live birth. This recommendation is pertinent to the whole world, where the majority of population lives in the middle- and low-income countries. It may not be applicable to high income countries where adequate nutrition and appropriate health care are universally available. Previous research on the subject produced inconsistent results with some studies demonstrating increased risk of adverse outcomes if the InterPregnancy Interval (IPI) was either too short or too long, while other studies indicated that there is no difference. The study by Tessema et al investigated the impact of short (<6 months) and long (>60 months) IPI on two adverse outcomes, namely preterm birth (either medically induced or spontaneous) and babies being born small for gestational age. The study included almost 4 million women who gave birth to 5.5 million children (between-women analysis). Some women had more than two children, and these were included in a separate analysis where IPI was assessed in terms of adverse outcomes in individual women (within-women analysis). All women gave birth in high income countries, so the results may not be directly applicable to all countries.
The main results indicate that the risk of spontaneous (not medically induced) preterm birth may be increased in women who conceive less than 6 months following a live birth. This increase in the risk of preterm birth was in the order of about 40% and was significant, both statistically and likely clinically. Both preterm birth and smaller babies were more common in the whole groups of women who conceived <12 months and >24 months after their last live birth. Importantly, all adverse outcomes (SGA and PTB) were more common in women who waited to conceive for more than 60 months following the birth of their last child.
There are several theories to explain these findings. The most common explanation is that a short interpregnancy interval does not allow repletion of minerals and nutrients that are used by women to nourish a pregnancy and are depleted during childbirth. This explanation does not appear to be relevant in high income countries where true nutrient deficiencies are extremely rare and adequate diet allows for complete replenishment in a short period of time. On the other hand, this is an important consideration in middle- and low-income countries and is the basis of the WHO recommendation to delay pregnancy for at least 24 months. There are other considerations which may explain the association between short IPI and adverse pregnancy outcomes, such as shorter interval to recover from various complications associated with giving birth and reduced ability of women to reduce their weight before the next pregnancy. Both increased weight in a subsequent pregnancy and sequelae of birth complications may increase the risk of adverse pregnancy outcomes, independent of IPI.
The increased risk of adverse outcomes associated with longer IPI (>60 months) may be attributed to ageing, decreased uterine adaptability, increased risk of maternal chronic illness and the impact of infertility. Prolonged IPI may not be voluntary, in a sense that women who did not have another pregnancy within 60 months following a live birth may not have waited by choice. These women may have had infertility which in some cases may have required fertility treatments. Both infertility itself and its treatments are associated with adverse pregnancy outcomes. Unfortunately, this study could not ascertain the likely mechanisms behind its findings.
