What are the best times to become pregnant?

Hello, I am asking for a friend and have this info 3rd. hand. A lady had her menstrual cycle about March 1st.
then has sex bout March 14th. and about March 30th starts bleeding bad. She goes to Doctor who tells her she is pregnant. Is this a possibility or another problem?
Also what are the best times here to become pregnant?
I am doing this for a lady friend for her son.
Can you help?