Despite the fact that the Zika virus has quickly spread through the Americas and infected thousands of pregnant women, there’s still many unknowns about the virus. For instance, doctors still don’t know why some pregnant women are more likely to have a baby born with a Zika-related birth defect compared to others. But now a small new animal study is adding some clarity.

The report, published Tuesday in the journal Nature Communications, looked at monkeys infected with the virus. The University of Wisconsin-Madison and Duke University researchers found that the monkeys who were infected with Zika became immune to the virus weeks later, something that’s also thought to occur in humans. They virus also lasted longer in pregnant monkeys compared to non-pregnant ones.

Non-pregnant monkeys were virus-free after about 10 days of getting infected, but pregnant monkeys had virus in their bloodstream for between 3o to 70 days. Since the study was completed in monkeys, and in a small number of them (eight in total), the findings do not directly translate to pregnant women. However, the researchers argue their study offers proof of a concept, and suggests that monkeys are a good animal model for learning about Zika.

Understanding how Zika behaves in pregnant women is a priority, since that the virus can cause severe birth defects in infants. There are close to 300 pregnant women in the United States with a Zika infection, and in Colombia, scientists put the number at closer to 12,000. The long-term effects of Zika infection—as well as what other health complications it can cause—are unknown.

Why the virus remained in pregnant monkeys for longer isn’t shown in the study, but the researchers cite some possibilities. It’s possible that women who are pregnant have weaker immune systems, or it’s possible that when the virus enters the fetus’ bloodstream it increases the amount of virus in the mother’s bloodstream as well. The researchers suggest that if this is indeed happening, it may be possible for health providers to regularly measure the amount of virus in pregnant women to gain a better understanding of how their fetus might be impacted.

This article originally appeared on Time.com.