Pregnancy date and scan?

[Bradjeff]

Hi, my ex partner has PCOS and very irregular periods. She went for a check up yesterday due to stomach pains and was told she was pregnant. She was then sent for a scan where she was initially told she was 6 weeks pregnant. The tech then did an internal scan and said she was actually 7w1d. We last slept together unprotected on 18th sept. She had some bleeding on sept 20th (not sure if this was period or implant bleeding and pain now). We then had protected sex on the 29th sept. That was the last time we slept together. She had since told me she slept with someone during November. How accurate are the scans especially as they changed by a week? Is it possible the baby could be mine even though it would then mean she would be 14 weeks pregnant? Is it possible for the weeks to be confused during the scan or for 14 week baby to be developing very slowly? Please help!

FWIW if her LMP was 5/9/18 and you had unprotected sex on 18/9/18 there is an incredibly high chance this is your baby.

But there is no way a 14+ week scan wiuld resemble a 7 week foetus.

Yes you are right it is my ex partners baby and with all the evidence presented most likely not mine, it’s just I do still care. Maybe I’m being somewhat naive but it’s also a complicated situation.
Tbh I don’t think that was her official last LMP. With her having PCOS, they were very very irregular and she never properly kept track.
I think al the replies I’ve given and what I’ve read suggest that early scans are the most definitive and the dating one at 12 weeks will most likely be the date?
I’ve assumed that they measure and date the baby at each scan - this was the scheduled 20 week scan but I think it is a mistake somewhere on the dates. It’s just a complete mind mess though with everything.

Being very honest with you, I think you are torturing yourself by insisting on being involved with the pregnancy. She is your Ex! You need to close that book and move on. I understand it’s hard as presumably you love her but she is having someone else’s baby. Are you hanging around, trying to be involved because you hope she will think you’ll make a great dad and ask you to get back together and raise this baby? Thats what it looks like.

You need to let go.

After 16-20 weeks, I think, you can only measure bits of baby as the foetus will be too big to fit on the screen. The 12 week scan (or earlier) is used for dating because it's easy to measure a baby from top to bottom and the sizes of all embryos and foetuses are roughly the same at that stage. They only diverge later.

That said, your ex's lmp may have been on the 5th for real business nothing says she ovulated 14 days after.

What was the measurement for the Transcerebellar Diameter as that is considered the most accurate way to date a baby in later gestation - basically it should be the same in mm as the mother is in weeks.