One of my mother's group friends conceived their DD through IVF as they had been trying for 3 years and had 'unexplained infertility'. They wanted to try for a second and were heading off to the fertility specialist so did the pregnancy test before going and it was positive. She was 3 months pregnant and didn't realise - she thought she was tired as she had just returned back to work after mat leave.

Her take on it was that maybe her hormones had been messed up and by having the first baby her body sort of clicked as to what it should be doing and her hormones balanced out enabling her to concieve naturally the second time.