noun
pregnancy stretch mark; striae gravidarum
Medical and everyday term for stretch marks that appear on the abdomen, breasts, or thighs during pregnancy.
妊娠線を予防するためにクリームを塗っています。
I'm applying cream to prevent pregnancy stretch marks.
出産後、妊娠線が少し薄くなっ
After giving birth, the stretch marks faded a little.
ストレッチマーク is a loanword for stretch marks in general, used in beauty and medical contexts, while 妊娠線 is the native Japanese term focused on pregnancy-related marks.
Compound of 妊娠 (にんしん, 'pregnancy') and 線 (せん, 'line'), literally 'pregnancy lines'.