become pregnant
孕 centers on the state of carrying a child inside the body: becoming pregnant.
孕 combines 乃 (a pictograph of a pregnant woman's body) with 子 (child) to express the idea of pregnancy. The modern form preserves this visual metaphor.
The top 乃 looks like a rounded belly, and the bottom 子 is a child. Together they show a child inside a belly: becoming pregnant.
For ヨウ, imagine a woman saying 'Yo!' when she finds out she is pregnant: Yo -> ヨウ, and the belly shape 乃 holds the child 子.