When is it too soon – or too late – to have another baby?

Vancouver Coastal Health Research Institute (VCHRI) scientist Dr. Gillian Hanley’s latest study, done in collaboration with the Surveillance team at Perinatal Services BC, questions the research that informs the counselling and recommendations that many British Columbian women get about how long to wait after giving birth before getting pregnant again – also known as an interpregnancy interval (IPI). The World Health Organization (WHO), for example, recommends that women wait at least two to three years between pregnancies to reduce adverse birth outcomes, such as infant and child mortality, and to benefit maternal health1.

“The literature has really suggested that women avoid short IPIs [of less than 12 months] because it’s been reported that short IPIs are associated with pre-term birth, low birth weight, small-for-gestational-age birth, use of neo-natal intensive care unit, and even neo-natal deaths and congenital anomalies,” says Dr. Hanley, associate member of the Centre for Clinical Epidemiology and Evaluation (C2E2), a VCHRI research centre. “And waiting a long time [of longer than 48 months] also appears to pose similar risks.”

Short IPIs are thought to negatively affect newborns because of insufficient maternal recovery for the physiologic stress of the previous pregnancy and lactation. Long IPIs on the other hand are considered risky because of the loss of physiological adaptation from the last pregnancy and the mother having gradually returned to a state similar to before her first pregnancy.

Although these two theories are widely accepted, Dr. Hanley felt compelled to investigate IPI recommendations because the existing research is based on large administrative datasets that make it challenging to control for independent variables that are likely correlated with both IPIs and infant outcomes.

“Things like socio-economic status, social conditions, lifestyle factors, or access to care are obviously going to be hugely important for women planning interpregnancy intervals,” explains Dr. Hanley, who is also an assistant professor in the Department of Obstetrics and Gynaecology in the Faculty of Medicine at the University of British Columbia and an epidemiologist with Perinatal Services BC. “And that kind of data, which you really need to create a connection to infant outcomes, is hard to find within these large administrative database studies.”

“Here in B.C. we luckily have data from women who’ve had more than three births in our province over the last 14 or 15 years; those women can actually act as their own controls,” Dr. Hanley says of the research endeavour. “So we can see whether or not infant outcomes in one woman’s births differed based on the different IPIs that she had between her first and second births, and then second and third births.”

“We’re doing this because one woman who has given birth multiple times will likely be more similar in the factors we want to control for, such as socioeconomic status, social conditions, and lifestyle, than comparing across different women,” she explains. “Because all of the previous literature compared women against other women who had different IPIs, those studies were unable to adequately deal with such issues.”

B.C.’s older first-time moms feel pressure weighing the risks and benefits of waiting

B.C. women tend to wait longer than other Canadian women before having their first child. Because there are more older first-time moms in the province, providing accurate information about the risks of different IPIs becomes critical for those wanting to have more children later in life.

“Older women really want to understand this issue with short IPIs because they also feel like they’re running up against increasing maternal and infant risks if they wait too long before having another child,” says Dr. Hanley. 

“So, women are experiencing a trade-off between the possible adverse effects of too short an IPI versus the adverse effects of waiting out the IPI and having another child when they’re older.”

“We hope to improve upon the research in an attempt to make these recommendations around IPIs clearer,” Dr. Hanley says. “And we hope to achieve this by trying to remove some of these underlying issues that we think are problematic in the research that has been done to date.”