The chief midwifery officer for England is urging expectant mums to have their Covid-19 vaccination as soon as possible.
There has been a worrying rise in unvaccinated women being admitted to hospital with severe Covid-19, and evidence that the Delta variant poses a significantly greater risk than all previous strains.
Pregnant women are particularly vulnerable to becoming severely ill from Covid-19, and about one in 10 admitted to hospital with symptoms of Covid-19 require intensive care.
One in five pregnant women admitted to hospital with symptoms of Covid-19 gives birth prematurely.
"It is very concerning that admissions of pregnant women to hospital with Covid-19 are increasing and that pregnant women appear to be more severely affected by the Delta variant of the disease," said Marian Knight, professor of maternal and child population health at the University of Oxford, and chief investigator of the study.
"Around 200 pregnant women were admitted to hospital with Covid-19 last week. I cannot emphasise more strongly how important it is for pregnant women to get vaccinated to protect both them and their baby."
New data from the UK Obstetric Surveillance System shows that Covid-19 vaccinations offer effective protection from these risks.
The new analysis reports information on all pregnant women admitted with symptoms of confirmed Covid-19 in the UK from the start of the pandemic to July 11, 2021.
More than 99 per cent of pregnant women admitted to hospital with symptomatic Covid-19 are unvaccinated.
In comparison, 60 per cent of the general population admitted to hospital with Covid-19 have not been inoculated.
Since February 1, at least 55,000 pregnant women have received one or more doses of a Covid vaccine in the UK.
The study, funded by the National Institute for Health Research, showed that women admitted in the Alpha coronavirus period were more likely to require respiratory support, have pneumonia and be admitted to intensive care, compared to women admitted in the first wave.
Women admitted during the Delta period had a further increase in risk compared to those during the Alpha wave, with a greater proportion having pneumonia.
Babies born to mothers in the Alpha period were more likely to require admission for neonatal care compared to the first wave.
A similar proportion of babies born to mothers in the Delta period were admitted for neonatal care compared to the Alpha period, but more than half of the mothers admitted during the Delta period have yet to give birth.