Pregnancy & prenatal testing: Third trimester

Group B streptococcus is a bacterium that is present in the vagina and gastrointestinal areas of 10% to 30% of pregnant women, though it rarely causes an infection. If an infection does develop, the bacteria may infect the uterus, amniotic fluid, urinary tract, and any incision made during a caesarean section. At delivery, when the baby passes through the birth canal, the bacteria can spread to the baby and cause serious infection in the newborn.

Some centres routinely screen all pregnant women for Group B streptococcus at around 36 weeks, by taking a vaginal and anorectal swab. Other centres do not screen, but treat all women who have an increased risk of passing this infection on to their baby. This includes those with premature rupture of membranes (waters broken early), prolonged delay between rupture of membranes and delivery, those with a raised temperature during delivery, those who have had a previous baby infected by Group B streptococcus and those who have had Group B streptococcus in a urine test. The recommendation for these women is to receive antibiotics intravenously during labour.

Limitations of Group B strep screening
The available screening tests may not detect some women with Group B streptococcus. Labour may begin before the test results are available.

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