New data has revealed unvaccinated pregnant women make up nearly 20 percent of the most "critically ill" COVID-19 patients in England.
Health officials are now urging pregnant women to get a COVID-19 vaccine.
The NHS said since July, one in five COVID-19 patients receiving treatment through a special lung-bypass machine were expectant mothers who have not had their first jab.
Some pregnant women are being treated with Extracorporeal Membrane Oxygenation (ECMO), a therapy used only when a patient's lungs are so damaged by COVID that a ventilator cannot maintain oxygen levels.
Of the women in England between the ages of 16 and 49 on ECMO in intensive care, pregnant women make up almost a third (32 percent). This is up from just 6 percent during the beginning of the COVID-19 pandemic in March 2020.
Top health officials now say the figures show just how important it is that expectant mothers get the vaccine to keep themselves and their babies safe.
"We are urgently calling for all pregnant women to come forward for their vaccinations," Dr Edward Morris, president of the Royal College of Obstetricians and Gynaecologists, said.
"There is robust evidence showing that the vaccine is the most effective way to protect both mother and baby against the possibility of severe illness from COVID-19. The disproportionate number of unvaccinated pregnant women in intensive care demonstrates that there is a significant risk of severe illness from COVID-19 in pregnancy.
"We do understand women’s concerns about having the vaccine in pregnancy, and we want to reassure women that there is no link between having the vaccine and an increased risk of miscarriage, premature birth, or stillbirth."
Jacqueline Dunkley-Bent, the chief Midwifery officer for England, said it was a "stark reminder" of the need to get the COVID-19 jab.
"You can receive vaccination at any time in pregnancy, but the risks that unvaccinated pregnant women face of becoming severely unwell if they catch COVID-19 show exactly why we advise you to do so as soon as possible."
So far, more than 81,000 pregnant women have received their first dose of vaccine, and around 65,000 have also had their second dose.
