A lot of women, and some men, claim they know when their child was conceived. Is this possible?
Yes.
Here is one reason:
First released sperm is the older, weaker, often dead sperm. Not in every intercourse does all the sperm, including the new one, get released.
There is something about the dynamics about the intercourse that causes it.
In general:
Nature has its way to let people more likely to stay together have a better chance at having kids.
(Sorry, I am not good with such topics, if you can explain it, after understanding, better, feel free to do so in a comment)