From Genesis 38 (KJV, with interpolated Hebrew):
2 And Judah saw there a daughter of a certain Canaanite, whose name was Shuah; and he took her, and went in unto her.
3 And she conceived (ותהר), and bare a son; and he called his name Er.
4 And she conceived again (ותהר עוד), and bare a son; and she called his name Onan.
5 And she yet again conceived (ותסף עוד), and bare a son; and called his name Shelah: and he was at Chezib, when she bare him.
As you can see from the Hebrew (full selection not included due to poor bidirectional skills), verse 5 uses the word ותסף (to repeat?) instead of the word for conception, ותהר. Why?