Women who had a caesarean when fully dilated should have their cervix closely monitored during subsequent pregnancies, say experts.

Emergency caesarean sections performed when a woman is fully dilated during labour may raise the risk of premature births in later pregnancies.

A study of hundreds of Australian women who had two consecutive births between 1989 and 2015 found caesareans at full dilation was associated with a two-fold risk of a subsequent spontaneous preterm birth.

The findings are published in the Australian and New Zealand Journal of Obstetrics and Gynaecology.

Since premature births pose a risk to newborn babies, the researchers say mums who have caesareans at full dilation - when the cervix has dilated to 10cm - should be closely monitored.

"Preterm birth is a multifaceted public health concern for which many risk factors are still unknown," says study co-author Dr Joanne Ludlow, an obstetrician and gynaecologist at Royal Prince Alfred hospital.

"Our study has shown a statistically significant two-fold increase in the rate of spontaneous preterm birth among women who had a fully dilated caesarean section compared with caesarean section in the first stage of labour," Dr Ludlow said.

The cervix is the lower part of the uterus and connects the uterus to the vagina. During pregnancy its job is to stay closed and only open at the end of the pregnancy.

When the cervix has become fully dilated in labour it kind of merges with the uterus and a caesarean runs the risk of causing potential trauma to the structure because it's harder to identify, explains RPA obstetrician and gynaecologist Dr Bradley d Vries.

"When you do a caesarean section when the cervix is fully dilated sometimes when you do the cut in the uterus it might extend down to the cervix and then that could cause some damage to the cervix. So in the future pregnancy it might not function so well," Dr d Vries said.

To investigate the risks it may cause for future pregnancies researchers conducted a retrospective cohort study of 2672 women done over a 25-year period (1989-2015) who had an emergency caesarean section.

Eighty per cent of these women (2142) had a caesarean section during the first stage of labour and 20 per cent (533) at full dilation of the cervix (10cm).

The rates of spontaneous preterm birth in a subsequent pregnancy were 1.7 per cent and 3.8 per cent, respectively.

While pre-term births were twice as common among those women who'd had a previous caesarean at full dilation, the overall risk is small, says Dr d Vries.