Scottish researchers found that women who catch the virus near the end of pregnancy were more vulnerable to birth-related complications.
They are more likely to suffer them than women who catch Covid in early pregnancy or not at all.
The researchers say getting vaccinated is crucial to protect pregnant women and their babies from life-threatening complications.
The latest findings come from the Covid in Pregnancy Study (Cops), which carried out research across Scotland to learn about the incidence and outcomes of Covid-19 infection and vaccination in pregnancy.
It is one of the first national studies of pregnancy and Covid.
The research team included scientists from the Universities of Edinburgh, Glasgow, Aberdeen, Strathclyde, and St Andrews along with Public Health Scotland (PHS) and and Victoria University of Wellington in New Zealand.
They found that preterm births, stillbirths and newborn deaths were more common among women who had the virus 28 days, or less, before their delivery date.
The majority of complications occurred in unvaccinated women.
The results, which have been published in Nature Medicine, come after recent data showed 98% of pregnant women admitted to UK intensive care units with coronavirus symptoms were unvaccinated.
Researchers are now calling for measures to increase vaccine uptake in pregnant women.
The study analysed data relating to all pregnant women in Scotland. It included more than 87,000 women who were pregnant between the start of vaccination uptake in December 2020 and October 2021.
Just 32% of women who gave birth in October 2021 were fully vaccinated, compared with 77% of the general female population aged 18 to 44.
All of the women whose babies died had not been vaccinated against Covid at the time of infection, though experts stressed that it is not possible to say if the virus contributed directly to the deaths or preterm births as they did not have access to detailed clinical records for individual women.
BBC