Twelve pregnant women with Covid-19 have been treated by the Belfast Trust in the past week.
RTÉ News understands that none of the women were vaccinated against the virus.
The Trust operates services for several hospitals, including Royal Victoria Hospital, Belfast City Hospital and the Mater Hospital.
In a statement today, the Belfast Trust said that pregnant women with the virus are having to be ventilated, "with the majority being hospitalised in their third trimester, over 26 weeks".
Pre-term deliveries have also had to be arranged for a number of pregnant women in recent weeks.
The Trust said that unvaccinated pregnant women are more likely to become infected with Covid-19, more likely to become unwell if they are infected, and more likely to require hospital admission and respiratory support.
It said that pregnancy can reduce an expectant woman's lung capacity, and Covid-19 can further increase pressure on the lungs.
The Belfast Trust said it has recently had to arrange pre-term delivery for a number of pregnant women and in these instances, their babies required admission to neonatal intensive care.
This has led to increased pressure on the system capacity, as babies with the virus have to be isolated for 10 days after they are born.
There are currently six babies in isolation.
"If a baby is in the neonatal unit and its parents are Covid positive, unfortunately - for infection prevention and control reasons - parents will not be allowed to hold their baby until they have completed the required period of isolation or had a negative PCR test."
Meanwhile the Health Service Executive has said that all maternity hospitals in Ireland are encouraged to discuss getting the Covid-19 vaccine with pregnant women when they attend for their first visit.
This booking visit usually occurs at 12 to 14 weeks' gestation.
In a statement, the HSE said: "Due to the cyber-attack, the provision of an accurate figure on how many women have had a consultation about a Covid-19 vaccine since May 24 to the latest available date is difficult.
"The HSE is not involved in a formal surveillance programme about pregnant women and Covid-19 vaccine.
"Information was initially collected when there was an imperative to urgently progress the vaccination of women over 30 weeks' pregnant.
"This was to ensure they could get their second shot before the 36 week cut off. We are now vaccinating from 14 weeks gestation."
The HSE said women are encouraged to talk to their obstetrician, midwife or GP about getting their Covid-19 vaccine if they are pregnant.
Once they have delivered their baby, they can register on the portal for a vaccine.
The Royal College of Midwives in Northern Ireland said there are increasing numbers of pregnant women being admitted to hospital with serious illness - "almost all of whom are unvaccinated".
It said that vaccination "remains the best way to protect yourself and your baby against Covid-19".
"Pregnant women are at greater risk of serious illness if they get Covid, and those with severe Covid are twice as likely to experience a stillbirth and three times as likely to have a preterm baby".
The Belfast Trust has urged pregnant women who are worried about the safety of Covid-19 vaccines to speak to their midwife, GP or hospital consultant for the "most up to date scientific position on Covid-19 vaccines".