The rise occurred in areas with confirmed local transmission, according to a new CDC report. 

Signaling pathways triggered by the mother’s immune system may cause complications during fetal development.

Some vaccine developers are taking steps to include them, in line with bioethicists' urging, but it will likely take years before any expectant mothers are enrolled.

The first-of-its-kind study follows on the heels of ethicists’ urging vaccine developers to conduct clinical trials in pregnant women.