In a worrying development, more than 150 pregnant women in the United States have been infected with the Zika virus based on new numbers from the Centers for Disease Control and Prevention.

That doesn’t count the addition 120-plus women affected by Zika in the U.S. territories, mostly in Puerto Rico, according to the CDC report.

Although there haven’t been many reports of pregnancies with complications, officials also don’t have full information on the situation, and it is a developing situation that could change quickly.

Zika virus, while relatively mild in terms of symptoms for adults who contract it, can cause serious birth defects or even lead to a miscarriage.

Scientists don’t yet fully understand what the exact risk is of the Zika virus, which is another cause for concern for officials. Another worrying reality is that officials are finding that women don’t have to be showing symptoms for their babies to be affected by the virus — just 49 percent of the pregnant women in U.S. states with the virus showed symptoms.