Pregnant patients who tested positive for the coronavirus were found to have more than double the risk of poor outcomes including preterm birth, blood clots, according to a study that emphasised the need to promote maternal vaccination.

The study, published in JAMA Internal Medicine, showed that COVID can also result in severe maternal morbidity, which includes conditions such as acute respiratory distress syndrome and sepsis.

"These findings add to the growing evidence that having COVID-19 during pregnancy raises risks of serious complications," explained lead author Assiamira Ferrara, a senior research scientist and associate director of the women's and children's health section at US-based non-profit Kaiser Permanente.

"Coupled with the evidence that the COVID-19 vaccines are safe during pregnancy, these findings should aid patients in understanding the risks of perinatal complications and the need for vaccination," said Ferrara. "This study supports the recommendation for vaccination of pregnant individuals and those planning conception."

The team analysed records for 43,886 pregnant individuals during the first year of the COVID-19 pandemic and found that the 1,332 who had a coronavirus infection during pregnancy had more than double the risk of negative outcomes compared with individuals without the virus.

The study found twice the risk for preterm birth for those testing positive for coronavirus. These patients were more likely to have a medically indicated preterm birth than a spontaneous one; risk was elevated for both types of preterm birth and during early, middle, and late terms of the pregnancy. Birth may be induced early when the mother has a condition such as preeclampsia - characterised by high blood pressure.

Those with COVID infection were also three times more likely to have thromboembolism or blood clots, and 2.5 times more likely to have severe maternal morbidity.

"Our study was large, diverse, and supports the need for vaccination by pregnant individuals and those who plan to get pregnant," said co-author Mara Greenberg, a maternal-foetal medicine specialist with The Permanente Medical Group. "The most important thing people can do to protect themselves and their baby is to get vaccinated."

The analysis found that 5.7 per cent of patients with a coronavirus infection during pregnancy had a hospitalisation related to the infection.