California, Texas, Florida, and even New York. The Zika epidemic has arrived on US shores. And the virus is already wreaking havoc on the lives on pregnant women.

This summer, several babies born in the US have been diagnosed with Zika-related microcephaly and one of them has died because of it. While we’re able to identify women and babies who test positive for the virus, there’s no definitive way to know whether these infants will have the developmental problems related to Zika.

Because this is the first Zika outbreak of this magnitude, much is still unknown about virus, its transmission, and the way it affects our bodies. The disease has been connected with serious pregnancy complications, including a higher risk of miscarriage and stillbirth, and neonatal conditions, such as microcephaly and other serious neurological and developmental conditions. It is believed that the risk of microcephaly may be in babies whose mothers contracted Zika in the first trimester of pregnancy, although mothers who contracted the virus during the third trimester gave birth to babies with normal-sized brains but were still born with cerebral damage.

The latest guidelines recommend testing for pregnant women who live in an area where Zika is endemic (which in the US is still limited) or women who have traveled to an infected area in the months prior to the pregnancy, even if they don’t present symptoms of Zika. Testing is also recommended for asymptomatic pregnant women whose partner has traveled to, or lives in, areas of active Zika transmission.

Robert Segal, a cardiologist and the founder of LabFinder, explained that a few commercially available tests have received a temporary Emergency Use Authorization (EUA) from the Food and Drugs Administration (FDA), but the FDA has yet to provide permanent approval, as well as specific guidelines for use.

The tests, Segal said, are at risk of giving false positive and false negative results—though he notes there are more false positives than false negatives—so sometimes one test isn’t be enough to give a final diagnosis. Women who have been exposed to the virus and show symptoms of Zika infection can take a polymerase chain reaction (PCR) test to look for the presence of Zika’s genetic material in the patient. This test is administered within the first two weeks of the onset of flu-like symptoms or a rash. After that window, another test is recommended that looks for signs of residual or previous infection and immune system response to Zika. If this test comes back positive, the CDC recommends performing a further antibody test to confirm the diagnosis.

If a woman has tested positive, the current recommendation is to wait at least eight weeks before becoming pregnant, and six months if her partner has tested positive for Zika. Yet it may be hard to determine exactly when a woman contracted Zika, if her infection was asymptomatic (which, Segal notes, happens in up to 80% cases). Doctors believe that a woman who tests negative weeks after exposure should be outside of the window of concern.

But what if a woman who is already pregnant tests positive?

The likelihood that a pregnancy would be affected, resulting in miscarriage or stillbirth, or that the baby would present a serious abnormality, is uncertain. Research has found the chances of developing microencephaly may be up to 13%, but the data set is still so small that nothing should be taken as definitive. For comparison, this is a slightly higher likelihood than whether a 49-year-old woman carrying a fetus will develop Down Syndrome.

Currently, it’s not possible to determine whether a fetus will have birth defects or lifelong problems because of Zika even at a more advanced stage of pregnancy, which makes it especially hard for women to contemplate terminating a pregnancy.

Clark Johnson, an assistant professor of maternal fetal medicine at Johns Hopkins University and a fellow with Physicians for Reproductive Health, says the recommendation for pregnant women who test positive for Zika is to increase the number of ultrasounds during gestation. In pregnancies that aren’t particularly risky, a woman usually undergoes two to five scans, the latest of which is around the 20th week. In the case of Zika, as well as other high-risk conditions, the recommendation is an ultrasound every four to six weeks, which is the time it takes for any significant fetal change to occur.

The scan, Johnson said, would look for anomalies in the shape and size of the head, which becomes visible around the 16th week or at times the 14th. Another warning sign may be an unusual deposit of calcium in the brain area. However, while it’s the best (and only) tool to diagnose Zika-related birth defects, Johnson said that “we don’t know that the ultrasound is perfectly diagnostic.” A woman can also test her amniotic fluid for Zika (a test usually performed around week 14 to 16) but that won’t necessarily give hard evidence on whether the fetus will have serious damage or not.

Similarly, it’s impossible to rule out conditions that are not visible. Some infants may have delayed development of the brain, which could happen in months after birth, or other problems that have been linked to Zika, such as trouble swallowing or moving. There could also be, Johnson said, babies with “normally sized heads but lower IQs” as a consequence of the infection—another thing that current diagnostics wouldn’t be able to reveal before birth.

As research on Zika progresses, improved testing will likely be able to deliver earlier and more accurate diagnoses. More data will help in understanding the likelihood of serious conditions for babies of Zika-positive women. This will put women in a better position to make informed decisions about pregnancies affected by the virus.