Babies born to mothers who contracted the swine flu virus faced a much greater risk of being stillborn, according to a new study.
Baby deaths among women infected with the 2009 strain of the virus were five times higher than normal.
There was also a greater risk of premature births when compared to mothers who had not caught the virus.
Health workers say the findings reinforce the message that all pregnant women should get immunised against flu.
The study was carried out by the National Perinatal Epidemiology Unit at the Oxford University and it examined the cases of every pregnant woman in the UK who was admitted to hospital while suffering from flu.
In 256 mothers infected with the H1N1 flu virus between September 2009 and January 2010, seven of the babies were stillborn and three died shortly after birth.
That is the equivalent of 39 babies in 1,000 dying, before or shortly after birth, compared to 7 in 1,000 in mothers not infected with the virus.
The research was led by Dr Marian Knight who said getting vaccinated against flu was the best protection.
“This new evidence of the risk to babies shows even more clearly the severe consequences H1N1 flu infection can have in pregnancy. By getting vaccinated against flu, women can prevent these risks to both themselves and their unborn child.”
Figures from the Health Protection Agency show that the uptake of flu vaccinations in England among pregnant women was relatively low.
Just over half of pregnant women considered to be in high risk groups, such as those with conditions like asthma, were vaccinated.
But only 36.6% of healthy pregnant women with no underlying health conditions were vaccinated.
Janet Scott, research manager at Sands, the stillbirth and neonatal death charity, says seasonal flu is not normally linked with stillbirth so it was alarming that there were much higher rates associated with H1N1.
“I suspect many pregnant women have no idea that flu could potentially be a serious risk to their baby, yet early immunisation is an easy and effective way for mums to protect themselves and their babies against the potential threat.
“These statistics are very worrying, and as the flu season approaches we would urge pregnant mums to go and see their GPs and get vaccinated.
“Furthermore, it is crucial that GP surgeries are equipped with the staff and vaccine stocks to be able to meet this need. These findings must not be ignored.”
Professor David Salisbury, director of immunisation at the Department of Health, urged all pregnant women to get the flu jab.
“This study shows how harmful flu can be for unborn babies. This is because pregnant women are more likely to develop complications if they get flu.
“The vaccine can be given safely during any stage of pregnancy. And studies have shown that mothers who have had the vaccine while pregnant pass some protection to their babies, which may last for the first few months of their lives. (Dominic Hughes/BBC News)