Pregnant women are supposed to be first in line for the swine flu vaccine, but they might have a long wait.

The nasal spray version of the vaccine, which became available first, is not approved for use in pregnant women because it contains a live virus. And many of the injectable vaccines can't be given to expectant mothers because of a controversial preservative used regularly in flu shots.

Several states, including Missouri, ban the use of the mercury-based preservative thimerosal in vaccines for pregnant women and children under 3. Illinois' law is even stricter, banning thimerosal in vaccines altogether based on public concerns about mercury toxicity and developmental disorders, including autism.

The federal Centers for Disease Control and Prevention says vaccines with thimerosal are safe for pregnant women and their babies. But state bans present a challenge to public health agencies trying to get a limited supply of vaccines to pregnant women who are most vulnerable to severe illness.

Missouri has so far received 4,200 doses of preservative-free H1N1 vaccine, but an estimated 60,000 pregnant women live in the state.

The vaccine laws need to be reconsidered for public health situations like the swine flu pandemic, said Dr. Sharon Frey, a lead investigator of H1N1 vaccine clinical trials at St. Louis University. About 25 pregnant women have received H1N1 vaccine through a trial at SLU.

"Laws are made for political reasons," Frey said. "There's no scientific evidence that shows that thimerosal causes autism."

There is evidence that swine flu is more threatening to pregnant women, who are nine times more likely to require hospitalization, according to a recent study from the New England Journal of Medicine. Nationally, 28 expectant mothers have died since swine flu first appeared in April.

Pregnancy makes a woman more susceptible to complications from flu because of changes to her immune system and diminished lung capacity caused by the growing fetus. The CDC put pregnant women at the top of the list for H1N1 vaccine, both to protect the mother and the baby, who can also gain some protective antibodies.

Anticipating shortages of mercury-free flu shots, Washington state's health department last month decided to temporarily bend their rules to allow pregnant women and children under 3 the choice of receiving H1N1 vaccines that have thimerosal.

Each flu season, Illinois' health department has to file an exemption to the state's thimerosal ban, allowing residents, including about 129,000 pregnant women, to get flu shots containing the preservative because they can't get enough supply.

"It's much more costly to manufacture a vaccine without the preservative in it, and obtaining that is difficult," said Melaney Arnold, spokeswoman for the Illinois Department of Public Health.

Thimerosal has been used for decades in vaccines to help prevent bacterial contamination. After concerns were raised about a possible connection between mercury toxicity and developmental disorders, including autism, the preservative was virtually eliminated from vaccines to quell the debate.

Federal health agencies have repeatedly denounced any link between vaccines and autism, and rates of the disorder continued to increase after thimerosal was phased out.

Thimerosal is still used in seasonal and H1N1 flu vaccines that come in multidose vials, which are cheaper to make and usually more plentiful than the preservative-free single-use syringes.

Most gynecologists and obstetricians in the area do not have any H1N1 vaccine and aren't making any promises to their patients. Two free clinics are planned later this month to vaccinate up to 2,000 pregnant women in St. Louis County.

"It's obviously limited. It seems to be a little bit of a problem right now," said Dr. James Bartelsmeyer, director of maternal-fetal medicine at St. John's Mercy Medical Center.

Vaccines can be a tough sell for pregnant women, who are taught to avoid many medications and foods. But Bartelsmeyer expects "a significant majority" of his pregnant patients to request the H1N1 vaccine.