Sforno on Genesis 38:24/ספורנו על בראשית ל״ח:כ״ד

1וגם הנה הרה, she did not even bother to hide her condition which reflects negatively on you. This is similar to a statement by our sages in Yevamot 35 “when a woman had illicit sex she engages in all kinds of bodily convulsions to avoid becoming pregnant and thereby revealing her shame and that of her husband or lover.”