England's chief midwife has stepped up her call for pregnant women to get the Covid jab as soon as possible.
Estimates based on GP records and Public Health England data suggest hundreds of thousands have not had the jab, as the number of mums-to-be in hospital with the virus rises.
Other data suggests the Delta variant increases the chance of severe disease.
In the last three months, 171 pregnant women with Covid needed hospital care - but none had had both jabs.
In a letter to midwives, obstetricians and GP practices, chief midwife for England Jacqueline Dunkley-Bent says all healthcare professionals have "a responsibility to proactively encourage pregnant women" to get vaccinated.
She recommends advice on jabs be offered at every opportunity.
"Vaccines save lives, and this is another stark reminder that the Covid-19 jab can keep you, your baby and your loved ones, safe and out of hospital," the chief midwife told pregnant women.
Since mid-April 2021, mothers-to-be have been offered the Pfizer or Moderna coronavirus jab, with the second dose recommended eight weeks after the first.
'Admitted every day'
Public Health England data suggests about 51,724 pregnant women have received one Covid vaccine in England so far. Of these, around 20,648 have had their second dose.
This is out of approximately 606,500 pregnant women in England in 2020-21, based on estimates from GP records.
While uncommon, severe illness with Covid-19 is more likely in later pregnancy.
UK Obstetric Surveillance data looking at pregnant women admitted to English hospitals up to July shows:
- The proportion admitted with moderate to severe Covid has increased with the Delta variant compared to previous strains
- In the last three months, 171 pregnant women were admitted to hospital with Covid symptoms
- Some 98% were unvaccinated and just three had received a single dose of the vaccine
- About one in three pregnant women in hospital with Covid-19 developed pneumonia
- About one in seven needed intensive care
- About one in five admitted to hospital with Covid go on to give birth prematurely and their likelihood of having a caesarean section increases
'I was nervous but feel safer and happier'
Yvonne O'Connor, who is 36 weeks pregnant and attending an antenatal clinic at Chelsea and Westminster Hospital in London, said she started getting worried about Covid in her third trimester.
"Luckily, I haven't had Covid at all," she said. "I've been very lucky that way.
"But I just thought it was important to have it [the vaccine] to protect me and the baby now as well. So I decided to do it."
At the same clinic, mum-to-be Amy Weir, 29, said she had discussed the risks and benefits of the vaccine with her midwife.
"I decided that... I was in a better position having the vaccine than I was potentially being exposed to the risk of Covid," she said.
Victoria Rawlings, whose baby is due in November, said she was "really nervous" before having the her first jab.
"And then I did a bit more reading and thought, actually, this is perfect," she said. "And I feel much safer, much happier, much freer to go out and about."
But it's not been a straightforward decision for everyone.
Iona Debarge, who is 30 weeks pregnant, said she was nervous because the vaccine was still very new.
"I think if it had been going for five years or something like that, I would feel more confident to get it done. That's why I'm still hesitating."
Lead researcher Prof Marian Knight, from Oxford University, said she was very concerned about the recent rise in pregnant women in hospital with the disease - though numbers are not as high as those seen during earlier peaks in the pandemic.
She said that during the winter wave, when the Alpha variant was dominant, one in 10 pregnant women admitted to hospital with Covid symptoms required intensive care - but now, with the Delta variant dominant, she said that figure was one in seven.
Prof Knight told BBC Radio 4's Today programme that pregnant women could be confident about having the vaccine, with the study showing it was "very protective" with no concerns over safety.
"Pregnant women were obviously naturally hesitant because there wasn't any evidence, but women can be reassured now because we have this real world evidence about the vaccine being used in practice," she said.
She added that by getting the vaccine, pregnant women would be protecting themselves and also passing protective antibodies onto their babies.
According to NHS England, more than 55,000 pregnant women across the UK have received at least one dose of the vaccine, with no safety concerns.
Dr Edward Morris, president of the Royal College of Obstetricians and Gynaecologists, said doctors were seeing very sick pregnant women with Covid-19 in hospital every day, with most being unvaccinated.
He added: "One dose of Covid-19 vaccination gives good protection against infection, so the sooner you can book your first appointment the better.
"You can have your second dose eight weeks after your first, which will provide a good level of immunity against the Delta variant."
Dr Sarah McMullen, of the National Childbirth Trust, said she was "extremely concerned" that many pregnant women remain unvaccinated and vulnerable, and strongly encouraged women to consider having the jab
She added: "It is understandable that pregnant women have questions and hesitations about vaccinations and they need to be able to trust in the information and support to make an informed decision.
"We've been really frustrated to hear of so much misinformation and the confusion this has caused."