The county warned of the recent tripling, reporting that 27 cases were reported during the week that ended June 27, while 81 were reported during the week ending July 25.
The County Department of Public Health is encouraging pregnant women and women who are breastfeeding to get vaccinated.
"Unfortunately, pregnant women are at high risk for serious health problems if they become infected with COVID-19," county Public Health Director Dr. Barbara Ferrer said in a statement. "Getting vaccinated is the best way to reduce the risks of COVID-19 infection and complications for both you and your baby. If you are pregnant or a new mom, we encourage you to get vaccinated as soon as you can and you can get vaccinated at any time during pregnancy. Studies continue to show the vaccines are safe for expecting and new moms and are very effective against COVID-19 and the Delta variant."
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As of Aug. 10, 11,264 pregnant women have tested positive for COVID, according to the county. A majority of the cases - 77% - have been among Latinas.
"Although rare, women infected during their pregnancy can pass on the virus to their newborn," a county statement said. "Among the 10,998 births where there was testing information, 55 infants tested positive for COVID-19."
Twelve pregnant women who tested positive have died.
A total of 2,426 new COVID-19 cases were reported Monday, bringing the overall case count to 1,352,791 since the start of the pandemic. Five deaths were reported and 1,653 people are currently hospitalized due to COVID.
Cases have been reported at an average rate of more than 3,000 cases a day for about the past week, and a rise in case numbers can be expected in the coming weeks with increased testing at schools and businesses, according to county health officials.
City News Service contributed to this report.