be(come) pregnant
姙 centers on the state of carrying a child: becoming or being pregnant.
姙 combines 女 (woman) with 任, which likely contributes the sound and may carry a sense of bearing or entrusting, together expressing pregnancy.
A woman 女 with a burden 任 inside her: she is carrying a child, becoming pregnant.
For ニン, imagine a pregnant woman knitting tiny clothes: knitting -> ニン, and she is 姙.