More than 4 million refugees have spilled out of Syria's war in recent years. Tens of thousands of them have braved deadly land and sea routes into Europe, where political leaders continue to struggle with the massive influx, but the overwhelming majority have stayed in Lebanon, Turkey and Jordan.

Many have lived in formal refugee camps at one point or another before venturing toward urban areas or into other countries. In Jordan, some now reside in informal settlements that have sprouted along its northern border with Syria.

A new series of portraits by Muhammed Muheisen -- the Associated Press's chief photographer for the Middle East, Afghanistan and Pakistan -- highlights the challenges faced by some those who are among the most vulnerable refugees: mothers. Muheisen, who has committed years to documenting the spiraling crisis, spoke with them during their pregnancies earlier this year and again when he returned in August. Both times, he took their portraits.

They told Muheisen of the hardships of childbirth in such a setting, of worries about what they'll feed their newborns, of feeling abandoned. One woman, a 20-year-old from Hama, said the birth of her first child, a son, made her feel "complete."

Read more: