Sperm can last 5-6 days in the woman's reproductive tract. But also keep in mind that your scans, if they are giving you a count like "x weeks pregnant," are beginning the count at the assumed first day of your last period, not the assumed day of conception.
Sperm will actually burrow into the lining of your uterus until an egg is released and as said above, can live for 5-6days like that so it is very possible for you to have conceived a few days after you had sex
And also keep in mind your due date is only a guess. And your drs go by your lmp. So really. If your 40 weeks, it's been 40 weeks since your lmp but baby is 38 weeks. Thats why it is normal for some woman to go to 42 weeks. And some sooner. Good luck!!
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