During the 2009 and 2010 flu seasons, millions of pregnant women received the vaccine against swine flu, or H1N1 influenza, yet but less than 300 possible complications were reported to a national database.
Researchers estimated that out of one million pregnant women who received the vaccine, 118 experienced a potential side effect from the shot.
These findings support the official recommendation that pregnant women receive the seasonal flu vaccine, which will contain the H1N1 vaccine in the upcoming flu season, said study author Dr Pedro Moro of the US Centres for Disease Control and Prevention.
"Based on all the information we have available, we definitely think pregnant women should receive the flu shot in the 2011-12 season," Moro said. "The flu shot will protect pregnant women, their unborn babies, and protect the baby after birth."
Compared with women of the same age who aren't pregnant, expecting mothers are more likely to become seriously ill from a flu infection and need hospitalisation.
According to the CDC, pregnant women accounted for one in 20 deaths from H1N1 influenza in 2009. By comparison, only one in 100 was pregnant in the population.
The latest findings stem from data submitted to the Vaccine Adverse Event Reporting System (VAERS), a vaccine-safety surveillance system run by the federal government. It allows anyone - including doctors, vaccine makers and patients - to report health problems that arise after a vaccination.
The system helps health officials spot new, unusual side effects, although with the caveat that the reported problems are not necessarily caused by vaccination.
Last year, a survey of reports to the VAERS found no unusual complications among pregnant women who've received the vaccine in the past 20 years.
In the latest report, published in the American Journal of Obstetrics and Gynaecology, researchers at the CDC and Food and Drug Administration reviewed health problems reported following the swine flu vaccine. Again, they found nothing unusual.
"We found no patterns suggesting a safety concern for the mother or the baby," Moro said.
For every one million women vaccinated during pregnancy, there were 49 reports of miscarriage, and eight reports of stillbirths.
Both of these serious events occur relatively commonly in pregnancy, according to the researchers. Nearly a quarter of pregnant women 34 and older experience miscarriage, while fewer than one in 200 pregnancies result in stillbirth.
"It is important to remember that some health events will happen by chance shortly after vaccination," said Moro. "For example, by chance alone you expect to find reports of miscarriages in pregnant women."
Pregnant women were no more likely to experience less serious complications, such as allergic reactions, following the vaccine than non-pregnant adults either.
Dr Michael Schatz, who studies vaccine safety during pregnancy but was not involved in the new work, called the findings "reassuring."
"No reasons exist from this report not to continue to follow the CDC recommendations and, pending more information, consider that the benefits of influenza immunisations in pregnant women outweigh the risks," Schatz, of Kaiser Permanente Medical Centre, said. (Reuters Health/ June 2011)