QUESTION: Hi sheva,

I think I know the answer but I just wanted another opinion. I had a stupid one night stand on 4th sept. Then I had my period on approx 13th or 14th sept, and slept with my partner 23rd sept and 28th sept. my 12 week scan dated me at 11w5 days on 4th dec, which is out by a day or two because I definitely remember having my period on the 14th sept. I don't think the baby could be the one night stand as that was at the beginning of the month and I had my period afterwards, but I'm just so paranoid! And I don't remember that period really, but I think it was normal. Should I just trust the accuracy of the scan, which suggests the father is my partner? My periods are usually 6 days and 20 days inbetween them. Hope this makes sense!!

ANSWER: Dear Antonia,
When they count weeks of pregnancy, they count the amount of time from your last period. That means that your scan that dates you at 11 weeks 5 days means your last menstrual period (called the LMP) started on September 14th, just as you said, so your dates are spot on and the scan is quite accurate. This means that when they say 11 weeks 5 days, your baby is actually only 9 weeks and 5 days old, since conception.
This means the baby is your partner's.
Best of luck,

---------- FOLLOW-UP ----------

QUESTION: Hi sheva,
Thank you so much for your answer, that is reassuring. I guess I was worried that my period on the 14th was implantation bleeding from the one night stand, as i hardly remember it (was a hectic time, moving house  etc), but therefore the 12 week scan would calculate me being further along wouldn't it? My Edd is 22 June. Does the scan measure the fetus and calculate how many weeks since conception regardless of what you say your lmp is?
Thanks again for your help.

Yes, if that had been implantation bleeding, you would have measured about 13 weeks, not 11+5.  The scan measures the age of the baby based on where it is up to in development. An 11 week fetus has developed to a certain point, a 12 week fetus is considerably further on. The earlier the scan is, the faster the baby develops, so the more accurate the scan would be. They use your LMP just as a guide point, but they base the age on the scan.
I'm glad I was able to help.
Good luck,


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