Researchers found that of nearly 1.3 million U.S. women who had blood work done during pregnancy, only 59 percent were tested for Chlamydia -- a common STD that can cause pregnancy complications or be passed on to newborns.

That's despite the fact that experts generally recommend pregnant women be screened for Chlamydia.

In addition, 57 percent of women in the study were screened for gonorrhea -- a test recommended for some pregnant women.

The U.S. Centers for Disease Control and Prevention (CDC) says that all pregnant women should be tested for Chlamydia at their first prenatal visit. The American College of Obstetricians and Gynecologists (ACOG) has said the same since 2007.

As for gonorrhea, the CDC and other groups recommend screening for pregnant women who are at increased risk. That includes women younger than 25 and those living in areas of the country where gonorrhea is common.

"As our study shows, there's a significant gap between the recommendations and actual practice," said Dr. Jay M. Lieberman, who is medical director for infectious diseases at Quest Diagnostics Inc. and worked on the study.

Quest is a diagnostic testing provider that operates labs throughout the U.S. The company offers STD testing and funded the new study.

Since not all pregnant women are advised to have gonorrhea testing, it's difficult to tell whether the 57-percent rate in this study is appropriate or not, according to Lieberman.

However, he said, even women who should be screened, based on guidelines, are sometimes not. Of pregnant women ages 16 to 24, 69 percent were tested for gonorrhea, the researchers report in the American Journal of Obstetrics and Gynecology.

Screening plays an important role in catching Chlamydia or gonorrhea because both STDs often have no symptoms, Lieberman pointed out.

"These two infectious diseases are easy to diagnose, and easy to treat and cure," he said.

But without screening, Lieberman added, many cases will be missed.

If left untreated, Chlamydia and gonorrhea can cause pelvic inflammatory disease, which can lead to infertility or an ectopic pregnancy -- a dangerous condition in which the fertilized egg grows outside the uterus.

Both STDs can also infect the baby during childbirth. Chlamydia can cause eye infections or pneumonia in newborns, while gonorrhea can lead to joint infections or serious blood infections.

A limitation of the current study is that it's based on pregnant women who had blood tests done between 2005 and 2008. And during those years, screening guidelines were in flux; the ACOG recommendation came out in 2007, for example.

"Clearly, this is still evolving," Lieberman said. But, he added, "there's also no evidence that screening rates have improved."

A 2009 CDC study, for example, found that only a minority of U.S. women who should be screened for Chlamydia actually had been. Screening is advised not only for pregnant women, but for certain other at-risk groups -- like women age 25 or younger (see Reuters story of April 16, 2009).

Exactly why some pregnant women are not tested as recommended is unclear. "Our data don't speak to that," Lieberman said.

For some women, access to prenatal care is a hurdle, he noted. But the data in this study all came from women already in prenatal care.

Lieberman suggested pregnant women talk to their doctors if they have not been tested for STDs or are not sure if they have had testing. Other STD tests done during pregnancy include ones for syphilis and HIV.

Sometimes, Lieberman noted, women are put off by the suggestion that they have STD testing. "But doing these tests is not a judgment on them or their behavior," he said. "We're just trying to do what needs to be done to make sure you're healthy and your newborn is healthy."

According to the CDC, about 100,000 pregnant women in the U.S. are infected with Chlamydia each year. Just over 13,000 have gonorrhea.

SOURCE: bit.ly/JK6FFC American Journal of Obstetrics and Gynecology, online April 30, 2012.

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