Women who develop symptoms of Zika virus disease should wait at least eight weeks before trying to become pregnant, while men with confirmed illness should wait at least six months before having unprotected sex, federal health officials said Friday in the most detailed U.S. guidance yet on planning pregnancies during the outbreak.

The recommendations from the Centers for Disease Control and Prevention reflect a widening concern that Zika, long viewed mainly as a mosquito-borne infection, can also be transmitted through sex and may threaten a developing fetus. They also expose how much remains unknown: researchers do not yet know how long infectious virus persists in semen, how often sexual transmission occurs or what share of infected pregnancies will result in serious fetal injury.

The agency’s new preconception guidance for women and their partners advises women with confirmed Zika disease or symptoms consistent with infection to wait at least eight weeks after symptoms begin before attempting conception. Men with confirmed disease or compatible symptoms should wait at least six months after onset before attempting conception through unprotected sex. For women and men who traveled to an area of active transmission but had no symptoms, the recommended waiting period is at least eight weeks after the last possible exposure.

CDC officials emphasized that the intervals are precautions, not precise biological thresholds. The recommendations are designed to reduce risk while scientists work with sparse evidence from a fast-moving epidemic. “These are complex, deeply personal decisions,” the agency said, urging couples—especially those living in areas where Zika is spreading—to discuss pregnancy timing with clinicians.

Different clocks for women and men

The unequal waiting periods arise from different evidence about the virus in blood and semen. In women, the eight-week interval is several times longer than the period in which viral material has generally been detected in blood after symptoms begin. Earlier CDC clinical guidance said viremia in symptomatic patients was expected to last about one week and found no evidence that a pregnancy conceived after maternal viremia resolves would face harm from a prior infection.

For men, the agency chose a much longer margin because Zika RNA has been detected in semen for at least 62 days after symptoms began, and infectious virus had been cultured from semen at least two weeks after onset. The CDC said the six-month interval was three times the longest period then documented for viral RNA in semen. Detection of RNA does not necessarily mean the virus remains capable of causing infection, but the absence of systematic studies left officials unwilling to draw a shorter boundary.

A companion CDC report on sexual transmission advises men with diagnosed or suspected Zika disease to use condoms correctly or abstain from sex for at least six months after symptoms begin. Men who traveled to an affected area without becoming ill should take those precautions for at least eight weeks after leaving. If a man lives in an area with active transmission, the precautions may need to continue while transmission persists.

Those recommendations build on six laboratory-confirmed U.S. cases of sexually transmitted Zika described by the CDC. All involved transmission from symptomatic men, and all occurred within three weeks of the men’s symptom onset. A separate CDC analysis of travel-associated cases warned that sexual transmission would probably be recognized more often as infected travelers returned to the United States.

Pregnancy risk drives a precautionary standard

For most adults, Zika infection is mild or produces no symptoms. The pregnancy concern is different. Evidence from Brazil and other affected countries has linked infection during pregnancy with microcephaly, a condition in which a baby is born with an abnormally small head, as well as other brain abnormalities, pregnancy loss and impaired fetal growth. Federal officials have not yet declared the relationship conclusively causal, but the accumulating clinical and laboratory findings have prompted progressively stronger precautions.

In January, the CDC issued interim guidance for pregnant women that recommended postponing travel to areas with active Zika transmission and offered testing to pregnant travelers with symptoms. In February, the agency described nine infected pregnant travelers in the United States. Among the reported outcomes were two early pregnancy losses, two elective terminations, one infant with severe microcephaly and two apparently healthy births; two pregnancies were continuing without known complications.

Those cases cannot establish the frequency of harm, and the small number makes broad estimates impossible. They nevertheless illustrate the range of outcomes confronting clinicians and families. The new waiting periods are intended to reduce the chance that a mother becomes infected at conception or early in pregnancy, when the developing nervous system may be especially vulnerable.

For a pregnant woman whose male partner has lived in or traveled to an affected area, the advice remains stricter: the couple should use condoms or abstain from sex for the entire pregnancy. The agency does not recommend Zika testing simply to determine whether it is safe for a couple without symptoms to conceive, because available antibody tests can cross-react with related viruses and a negative result may not exclude a recent infection.

Puerto Rico exposes the limits of individual advice

The guidance has its sharpest public-health implications in Puerto Rico, where mosquitoes are already spreading the virus locally. Couples there cannot end their exposure by returning from a trip, and delaying pregnancy may require sustained access to contraception rather than a short waiting period.

A CDC-led assessment released Friday estimated that about 138,000 women in Puerto Rico—roughly 22 percent of women ages 15 to 44—do not want a pregnancy and are not using one of the most effective or moderately effective contraceptive methods. The analysis of contraceptive needs cited high costs, inconsistent availability, limited provider training and reimbursement barriers as obstacles to intrauterine devices, implants and other methods.

The report called for broader access to the full range of contraceptives, including same-day availability of long-acting reversible methods, and for removal of administrative and financial barriers. It also stressed that counseling must remain voluntary and centered on each patient’s reproductive goals. The policy challenge is not merely to issue advice, but to ensure that people can act on it without coercion.

The University of Minnesota’s Center for Infectious Disease Research and Policy noted in its contemporaneous summary of the guidance that officials were balancing uncertain science against the potentially severe consequences of fetal infection. That balance is likely to shift as researchers obtain better data on persistence in semen, the incidence of sexual spread and pregnancy outcomes.

A moving target for clinicians

Doctors now must translate population guidance into decisions shaped by travel dates, symptoms, laboratory results, pregnancy plans and ongoing exposure. A person who develops fever, rash, joint pain or red eyes within two weeks of travel to an affected area may meet the clinical description of Zika disease, but most infections are asymptomatic. That makes the last possible exposure—not the recognition of illness—the relevant starting point for many couples.

The CDC describes the recommendations as interim and says it will revise them as new evidence emerges. For now, the eight-week and six-month intervals create clear markers in an outbreak defined by uncertainty. They also expand the response beyond mosquito control and travel warnings, making reproductive counseling, sexual-health precautions and contraceptive access central parts of the national effort to protect pregnancies.