Mums-to-be from BAME groups were more likely to be admitted to hospital with Covid-19.
The study also found that older women, those who were overweight or obese, and had high blood pressure and diabetes were more likely to be admitted to hospital.
But, the majority of pregnant women who did fall severely ill were in the later stages of their pregnancy - in the third trimester.
The Oxford University scientists say that indicates the importance for them to social distance.
The study, published as a pre-print, looked at 427 pregnant women admitted to hospitals in the UK between March 1 and April 14 with confirmed Covid-19 - 4.9 women out of every 1,000 pregnant women.
When looking at women from black and ethnic minority backgrounds, researchers found the inequality persisted even when women from London, the West Midlands and the North West were excluded from the analysis.
This indicates the difference cannot be explained by higher rates of coronavirus infection in those areas.
Women who were admitted to hospital with Covid-19 in pregnancy were less likely to smoke than a group of comparison pregnant women, the study also suggests.
The paper also reports that one in five babies born to mothers admitted to hospital with the virus were born premature and were admitted to a neonatal unit.
One in 20 babies born had a positive test for coronavirus, but only half of them immediately after birth, suggesting transmission of infection from mother to baby is low, researchers say.
Marian Knight, professor of maternal and child population health at Oxford’s Nuffield Department of Population Health, and lead investigator for the study, said: "A very small number of pregnant women do become severely ill with Covid-19 and sadly some women have died.
"Our thoughts must remain with their families.
"It is concerning that more pregnant women from black and minority ethnic groups are admitted with Covid-19 in pregnancy and this needs urgent investigation.
"Most pregnant women who were admitted to hospital were more than six months pregnant, which emphasises the importance of continued social distancing measures in the later stages of pregnancy.
"Following the current guidance about careful social distancing will help prevent infection."
Some 60% of the women admitted to hospital have now given birth, while the others have ongoing pregnancies.
Most of the women have now been discharged home, with around one in 10 women needing intensive care. Five of the women in the study have died.
Outcomes for babies born to mothers with Covid-19 were mostly good.
Although almost one in five were born prematurely and were admitted to a neonatal unit, fewer than 20 babies were born very premature - when their mothers were less than 32 weeks pregnant.
Gill Walton, chief executive of the Royal College of Midwives, said: "It's absolutely vital that women continue to attend antenatal appointments to ensure that they and their babies are well.
"Staying in touch with their maternity services team will help put any concerns at ease and enable them to act quickly when necessary."
Information for the study was collected from all 194 hospitals in the UK with a consultant-led maternity unit.
The study was conducted by researchers from Oxford Uni, in collaboration with the Royal College of Obstetricians and Gynaecologists, the Universities of Leeds and Birmingham, Kings and Imperial Colleges London.