Many couples achieve a natural pregnancy after having had an "test tube" baby. It happens in cases where the cause of infertility declines or disappears.
Stress is a good example. Some patients see their fertilizing potential reduced because of the stress caused when trying to conceive. Anxiety can affect egg and sperm quality, thereby hindering the odds of conception.
Making lifestyle changes (relaxing, changing dietary habits, diminishing or stopping the consumption of toxic substances, etc.) can help improve the chances of getting pregnant to a large extent.
Also, it may occur in cases of unexplained infertility.