expression
unborn child; baby one is expecting
A warm, everyday expression referring to the baby a pregnant woman is carrying. Often used in personal or family contexts.
お腹の子が元気に動いているのを感じた。
I felt the baby in my womb moving energetically.
彼女はお腹の子のために毎日歌を歌っている。
She sings to her unborn child every day.
胎児 is a medical/technical term for 'fetus', while お腹の子 is a warm, everyday expression used by expectant mothers and families.
A phrase composed of お腹 (polite form of 腹, 'belly') + の (possessive particle) + 子 ('child'). The exact origin is transparent; it is a natural, affectionate way to refer to an unborn child.