Around April 28th is when the baby would have been conceived. Bearing in mind though that ultrasound scans can be off by a couple of days but if the due date is around April 20th then April 28th is when the baby was made. This means that if you had sex up to about 5 days before the 28th that could have been when it happened, like StinaKaye said, sperm can live for up to 5 days in the right environment.
The time u conceieve is usually 10-15 days after the 1st day of your last period, but varies for every women.. So if ur trying to pinpoint a certain day bc ur a male and ur partner is now telling u she's pregnant and u don't thinks it's yours.. I would talk to her about it.. Not just turn ur back bc u don't think u layed ur sperm that day bc it could really b any day.. Sperm can live in a wet environment for 3 days I believe.. My 1st child I had unprotected sex on the 7th day and no more after that so that day was my conception date.. Even tho it didn't fall in the 10-15 day period of ovulation time the sperm lived in me and made connection with an egg during that time.. So keep that in mind.. Good luck with "your" pregnancy
Probably around the 28th July.
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