Adding to evidence that the flu shot is safe during pregnancy, a U.S. government study found no unusual complications among pregnant women who've received the vaccine in the past 20 years.

Researchers found that between 1990 and 2009, there were 175 reports of possibly vaccine-related medical complications among pregnant women submitted to the Vaccine Adverse Event Reporting System (VAERS).

That amounts to an estimated rate of 12.5 reported complications per one million pregnant women vaccinated against the flu.

Most were considered "non-serious." And while there were some reports of miscarriage and stillbirth, the numbers were substantially lower than the average rates of those complications in the general population.

VAERS is a vaccine-safety surveillance system run by the federal government; it allows anyone -- including doctors, vaccine makers and vaccine recipients -- to report health problems that develop after a vaccination.

Those reported problems are not necessarily caused by vaccination, so the VAERS system does not prove cause-and-effect. One of its goals is to help health officials spot new, unusual or rare side effects that may go undetected in the relatively small clinical studies done before and after a vaccine's approval.

The safety of the flu shot, which is made using killed influenza virus, has been studied in about 10,000 pregnant women, with no evidence that it presents a particular risk in this group.

These latest findings "add to the existing evidence that the (flu shot) is safe for pregnant women," lead researcher Dr. Pedro L. Moro, of the U.S. Centers for Disease Control and Prevention in Atlanta, told Reuters Health.

Public health officials, as well as medical groups like the American College of Obstetricians and Gynecologists, recommend that all pregnant women receive the flu shot -- though not the nasal-spray flu vaccine, which is made from a live, weakened virus.

Compared with non-pregnant women of the same age, expecting women are more likely to become seriously ill from flu infection and need hospitalization.

According to the CDC, pregnant women accounted for one in 20 deaths from H1N1 influenza (swine flu) in 2009. By comparison, only one in 100 was pregnant in the population.

That's why pregnant women are one of the "target groups" particularly encouraged to get the flu shot, Moro said.

Yet research indicates that no more than one-quarter of pregnant women in the U.S. got vaccinated during recent flu seasons. The reasons aren't clear, but doubts about the safety of the flu vaccine -- among both pregnant women and their doctors -- could be at work, Moro and his colleagues write in the American Journal of Obstetrics & Gynecology.

They found 148 reports of complications in the VAERS system among pregnant women who had received flu shots between 1990 and 2009. Another 27 came from pregnant women who inadvertently received the nasal-spray vaccine between 2003 and 2009 -- often because the women did not yet know they were pregnant or did not tell the health provider administering the vaccine.

Most of the reports -- 88 percent -- were classified as non-serious. Twenty percent of flu-shot reports and 59 percent of those related to the nasal spray contained no specific complication; instead, they simply indicated that a pregnant woman had received the flu vaccine.

Among the most common complications were skin reactions at the injection site, symptoms like fever and fatigue, and mild allergic reactions.

There were 20 reports of miscarriage, amounting to an overall rate of two per one million vaccinated pregnant women. By comparison, between 10 percent and 20 percent of U.S. pregnant women suffer miscarriages, with the rate varying by age.