Genesis 38:1

At that time Juda, twenty years old, marries the daughter of Sue, and has three sons by her during the three following years. The first takes Thamar to wife, when he was seventeen. Onan marries her the next year; after which she remains a widow about three years, when she bears twins to Juda. Phares goes down with him into Egypt, and has children there during Jacob's life. On this account, they are numbered among those who went down with Jacob, (chap. xlvi. 12,) as the children of Benjamin seem to be likewise. Thus all these events might happen during the twenty-three years that Jacob dwelt in Chanaan, and the seventeen that he sojourned in Egypt. Some have thought the time too short, and have concluded that Juda had been married long before Joseph's slavery. He was, however, only four years older. (Calmet)