Women who have Covid-19 are unlikely to pass on the infection to their babies during childbirth if precautions are in place, a small study suggests.
Of 120 babies born at three hospitals in New York, none tested positive for the disease after being born to infected mothers.
The results appeared similar two weeks later after some had been breastfed and shared a room with their mothers.
Experts say the results are reassuring but larger trials are needed.
Data on the risk of Covid-19 transmission during pregnancy and while breastfeeding is scarce, so recommendations for pregnant women and new mothers vary.
In the UK, the Royal College of Obstetricians and Gynaecologists suggests mothers should share a room with their babies and breastfeed if they wish, but with appropriate precautions.
Together with the World Health Organization (WHO), they say the benefits of breastfeeding outweigh the potential risks of Covid-19 spread.
Meanwhile, the US Centres for Disease Control and Prevention suggests considering a temporary separation of the newborn from a mother, to reduce the risk of spreading the virus to the baby. It says mothers in this situation may consider expressing breast milk.
In this study, mothers and babies were allowed to stay in the same room and mothers could breastfeed – with measures such as wearing face masks and frequent handwashing. Babies had enclosed cribs about 2m (6ft) from the mother’s bed.
All 120 babies were negative for coronavirus at birth when tested using nasal swabs.
82 were checked again a week later and all were negative. Most of these babies (68) had shared a room with their mothers and more than three-quarters were being breastfed.
72 babies were tested a fortnight after birth and found to be negative.
Researchers acknowledge almost a third of the babies did not have further tests after birth, partly because parents did not want to bring them back to a clinical environment during a pandemic.
Nevertheless, Dr Christine Salvatore, who led the study, said: “We hope our study will provide some reassurance to new mothers that the risk of them passing Covid-19 to their babies is very low.
“However, larger studies are needed to better understand the risks of transmission from mother to child.”
Prof Marian Knight, who leads the UK’s national surveillance of Covid-19 in pregnancy, said the research provided reassurance and supported current guidance.
She said: “More than 1,000 mothers with SARS-CoV-2 infection have given birth in the UK, and only 1-2% of their babies have had a positive test for SARS-CoV-2. Infection does not appear to cause severe illness in these babies.
“This small US study also indicates transmission of infection from mother to baby is uncommon with simple precautions such as the wearing of face masks by mums with Covid-19.”
The study is published in the journal The Lancet Child & Adolescent Health.