Infections in Pregnancy: Bacterial Vaginosis

What Do I Have?

Bacterial vaginosis (BV) is the leading cause of vaginitis (inflammation of the vagina). The other two major causes of vaginitis are candidiasis (yeast infection) and trichomoniasis (caused by a protozoan). BV affects over three million women annually, including about 60 percent of patients seen at sexually transmitted disease (STD) clinics. It is one of the most common genital tract infections in pregnant women, affecting approximately 12 to 22 percent of women.

What Causes BV?

Although BV is more common among women with STDs, no medical study has demonstrated that the infection is transmitted sexually. More likely, BV is the result of disturbances in the vagina's natural bacterial flora.

Like the mouth, intestines, and other parts of the human body, harmless bacteria normally inhabit the vagina. Many of these bacteria actually protect the body from infection by other bacteria that can cause disease. In the vagina, lactobacilli are the naturally occurring bacteria that ward off the proliferation of infectious bacteria. They typically account for 95 percent of the bacteria in the vagina. But, if the lactobacilli are reduced in number, other bacteria have the opportunity to overgrow.

Various factors may cause a loss of lactobacilli in the vagina-including use of antibiotics, vaginal medications, systemic hormones, contraceptive preparations, and douches, as well as sexual intercourse and STDs. A decrease of lactobacilli in the vagina, combined with the overgrowth of other bacteria (specifically, Gardnerella vaginalis and anaerobic bacteria, which grow in conditions of no oxygen), is the most common cause of BV.

What Are the Symptoms of BV?

Over 50 percent of women with BV do not show or have symptoms. Among those that do have symptoms, an unpleasant vaginal discharge is most common. The odor associated with this discharge is due to chemicals that are produced by the bacteria that cause BV. Menses (menstruation) and sexual intercourse usually worsen the odor because blood and semen chemically react with the bacteria to release odorous chemicals. The discharge is usually: (i) thin, (ii) dark, dull gray in color, and (iii) sometimes bubbly. Itching or irritation of the external vaginal area can occur.

What Are the Consequences of BV?

Untreated BV in non-pregnant women typically remains as a local inflammation, producing symptoms in the vagina only. BV in pregnant women was once regarded as a benign nuisance as well; however, at least nine recent clinical studies have shown that BV during pregnancy is associated with preterm labor, preterm delivery, and premature rupture of membranes (rupture of the bag of water prior to labor). Women with BV during pregnancy are about two to three times more likely to deliver preterm than women without BV. BV also is associated with an increased risk of intrapartum and postpartum infection (chorioamnionitis and endometritis).


Untreated bacterial vaginosis (BV) dramatically increases the risk of preterm delivery, the most common cause of complications and death in newborn babies.

Current scientific evidence suggests that the association between BV and adverse pregnancy outcomes is actually a cause-and-effect link. At least three studies show that treating BV in women at high risk for preterm delivery results in fewer preterm deliveries than in untreated women. The fact that a reduction in BV leads to a reduction in adverse pregnancy outcomes strongly supports a causal relationship. The extension of BV from the vagina into the uterus and amniotic fluid, combined with the local inflammation around the cervix, are two ways BV can potentially cause preterm labor.

In light of this evidence, the American College of Obstetricians and Gynecologists recommends that doctors screen for BV in pregnant women who are at high risk for preterm labor. Among other risk factors, if you have a history of preterm labor or weighed less than 110 pounds (50 kg) before your pregnancy, or both, you are considered at high risk.

How Is BV Diagnosed?

During an examination, your doctor will test for evidence of BV by:

  • examining vaginal discharges either directly or under a microscope;
  • checking the acidity of the discharge; and
  • detecting the odorous chemicals released by the bacteria causing bv.

The diagnosis is made most often by examining the discharge under the microscope and seeing the characteristic "clue cells" (cells that line the vagina and are covered with bacteria).

Each part of the examination can identify different characteristics of BV bacteria. When performed correctly, these methods accurately diagnose BV 90 percent of the time. In addition, a diagnosis of BV made during routine Papanicolaou (Pap) smears is considered accurate as well.

Once diagnosed with BV, patients who are adequately treated can expect a cure rate close to 80 to 90 percent.

What Are the Current Treatments for BV?

The 1998 guidelines from the Centers for Disease Control (CDC) recommend different treatment standards for pregnant and non-pregnant women with BV. (When reading these guidelines, remember that drug dosages vary; your doctor will prescribe what is right for you.)

Treatment for High-Risk Pregnant Women with BV:

  • BV should be treated with oral metronidazole (Flagyl) 250 mg, three times daily for seven days. Topical therapy does not decrease the rate of serious complications such as preterm delivery and, therefore, should not be used in high risk patients.

Treatment for Non-Pregnant Women with BV:

  • Metronidazole (Flagyl), 500 mg by mouth twice daily for 7 days;
  • clindamycin 2% cream (Cleocin Vaginal Cream), 5 g intravaginally, at bedtime for 7 days; or
  • metronidazole 0.075% gel (MetroGel Vaginal), 5 g intravaginally, once or twice daily for 5 days.

All three treatments have similar cure rates. The side effects are also similar, with the exception of oral metronidazole, which may cause severe nausea, vomiting, flushing, and headaches in the presence of alcohol. Intravaginal metronidazole has a lower reported frequency of gastrointestinal side effects than oral metronidazole because less of the drug is absorbed into the body from the vagina than from the intestinal tract. The topical forms of treatment are considered acceptable therapy in low-risk pregnant women, for example, women with symptomatic infection who are otherwise not at increased risk of preterm delivery.

Two alternative regimens are also acceptable:

  • metronidazole (Flagyl), 2 g orally, in a single dose (There is evidence that the single 2 g dose has a slightly lower cure rate, 69%, than the 7-day regimen described above, 72%); and
  • clindamycin (Cleocin), 300 mg orally, twice daily for 7 days (Oral clindamycin for 7 days is associated with more diarrhea than oral metronidazole).

The least expensive medication is oral metronidazole. Oral clindamycin is intermediate in cost, while the topical preparations are the most expensive.

Women who have been treated are usually not tested again unless their symptoms continue. There are no clear guidelines for treating the 10 percent of patients with BV who do not respond to the treatments discussed above. However, re-treating with a different antibiotic or extending the current treatment are both common approaches used in clinical practice. Finally, though sexual contact may be one cause of BV, partners of infected women are not usually treated.

What's New and Emerging?

As noted, clinical trials have shown that treatment of BV in certain high-risk populations can decrease the incidence of preterm delivery. On the other hand, current information does not yet support routine screening for BV in low-risk patients. The ability of BV to cause preterm labor in some women, but not in others, suggests that certain women are more susceptible to its effects. But current medical knowledge does not allow physicians to clearly identify which women have this natural predisposition. Because this information is not available for women having their first baby, it is impossible to determine the risk-or the need for treatment-in this group of patients. Unfortunately, the only way for women to reveal their own degree of susceptibility is by experiencing at least one episode of preterm labor. Future research to identify the factors that influence a woman's susceptibility may allow for treatment of at-risk women in their first, as well as subsequent, pregnancies.

Read This Next

Infections in Pregnancy: Yeast Infection
Infections in Pregnancy: Hepatitis
Sexually Transmitted Disease Symptoms You Should Know About
Emergency Contraception and Safety
Surprising Benefits of Probiotics

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