Daily valacyclovir taken from 36 weeks of pregnancy lowers the chance of a genital herpes outbreak at delivery, and with it the chance of a cesarean done for herpes. In a randomized trial of 350 women with a history of genital herpes, suppression cut herpes-related cesareans from 13 percent to 4 percent (Sheffield 2006).
Why the last month is when it counts
A baby faces the highest risk of catching herpes when the virus is active in the birth canal during a vaginal delivery. Neonatal herpes is rare, but it can be severe. Standard obstetric practice is to check for herpes lesions and prodrome symptoms in labor, and to deliver by cesarean if either is present. That single rule is why an outbreak at the wrong moment reroutes the whole delivery.
Suppressive antiviral treatment addresses the problem before labor starts. A woman with recurrent genital herpes takes a daily pill through the final weeks so the virus is less likely to reactivate at delivery. The timing target is 36 weeks, which places the drug on board through the window when labor can begin.
What the trial measured
The clearest evidence comes from a double-blind trial that assigned 350 pregnant women with a history of genital herpes to valacyclovir 500 mg twice daily or placebo from 36 weeks until delivery (Sheffield 2006). Two outcomes moved in the same direction.
| Outcome at delivery | Placebo | Valacyclovir 500 mg twice daily |
|---|---|---|
| Recurrent herpes requiring cesarean | 13% | 4% (P = .009) |
| HSV detected by viral culture | 9% | 2% (P = .02) |
| Neonatal herpes cases | 0 | 0 |
Suppression lowered both the outbreaks that force a cesarean and the amount of virus shed at delivery. No infant in either group developed neonatal herpes, and the trial was not large enough to measure that rare outcome directly, so the case for suppression rests on the reduction in recurrences and shedding rather than on a proven drop in newborn infections. Maternal and obstetric complications did not differ between the groups (Sheffield 2006).
The pregnancy dose is not the everyday dose
Suppression in pregnancy uses valacyclovir 500 mg twice daily, which is a heavier schedule than the 500 mg once daily used for ongoing herpes suppression outside pregnancy. Twice-daily dosing was chosen to keep drug levels steady through late pregnancy, when the kidneys clear the drug faster. A pharmacokinetic study confirmed that valacyclovir at this dose produced higher and steadier active drug levels than acyclovir, was well tolerated, and did not build up in the fetus (Kimberlin 1998).
Two different herpes questions in pregnancy
The transmission question people usually read about is a separate one. Daily valacyclovir given to a partner who carries HSV-2 cut the risk of passing the virus to a susceptible partner by about 48 percent (Corey 2004). That number describes sexual transmission between adults, not transmission from a mother to her newborn. Pregnancy suppression aims at the second question, and its evidence is the recurrence and cesarean data above.
Both uses share one fact worth stating plainly: suppression in pregnancy is a decision made with the clinician managing the pregnancy, not a self-directed refill. An outbreak at term, a first-episode infection during pregnancy, or a partner's status can each change the plan, and those calls belong in prenatal care. The daily suppression that Open Scripts covers is for non-pregnant adults, and its intake stops and refers when someone is pregnant.
The bottom line
For a woman with recurrent genital herpes, valacyclovir 500 mg twice daily from 36 weeks reduces outbreaks at delivery and lowers the shedding that drives a herpes cesarean, from 13 percent down to 4 percent in the main trial (Sheffield 2006). The drug is well studied at this dose in late pregnancy (Kimberlin 1998). It does not erase the risk of neonatal herpes, and it belongs inside prenatal care where the timing and the delivery decision are managed together.
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This article is for education and is not a substitute for individual medical advice from your own clinician.