Thanks for the query to ATD.
As her sexual encounter with her ex was on March 14 which was her first day of menses,then there are no chances of pregnancy from him as she was menstruating and in her safe period when ovulation had not occurred. She would have ovulated around 28 March and that was when she has been with you since, so it is more likely that you are the father of the child and not her ex.
I hope I have answered your query in detail.
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