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Need help with dates...mathematician required?

(6 Posts)
lilstar Tue 21-May-13 19:17:57

Knowing the date of my LMP was 27/02/13 but having had 2 scans so far, I am confused about my due date. Going by LMP it would be 04/12 .

I had a private scan which dated my at 6wk6days and tied in exact with my LMP at the time and when we DTD 11/03. Yesterday I had my NHS dating scan which they dated me at 12w5d (I was expecting to only be 11w5d). This has brought my due date forward to 27/11.

I am wondering if my scan yesterday is inaccurate and wondering when I actually got pregnant. We previously DTD on 14/02, my period before that was 30/01, so I really don't think I could be pregnant that early.

Would be greatful to anyone who is more knowledgable on the subject because I am totally confused and I know other people are far more conciencious about working these things out when they have a planned pregnancy but mine was unplanned. Thanks in advance for any replies.

jennimoo Wed 22-May-13 06:29:05

Scan dates are estimates based on size of the baby, so the dates don't always 'add up'.

I was tracking ovulation so know exactly when I should be due, but 8 week scan brought Edd forward half a week and 12 week scan made it a total of a week earlier! I've a friend who had ivf who therefore knows exactly when the embryo implanted, and get dates were changed at 12 week scan.

Not that the due date means anything anyway: the baby doesn't know when it's due!

willitbe Wed 22-May-13 12:46:06

Scans from 11 to 13 weeks are generally the only ones that are accurate to use for dating purposes. Earlier scans are very dependant on the angle of the uterus and prone to mis-measurement due to the small size being measured. Later scans are inaccurate due to babies growing at different rates and genetically being bigger/smaller etc.

It is strange how your earlier scan appears more to tie in with your LMP dates, so a bit odd. However as the pp has said, really babies never listen to due dates and arrive when they want to!!!!!

If I have it right your period was 27 Feb, and the only dates that you DTD was 14 feb and 11 march.

With 14 feb and having your period on the 27 feb you cannot, be pregnant from the 14th.

If your scan giving the due date of 27 nov is correct (as it should be within 3 days), then your assumed ovulation would be 6th March, so therefore since conception has to occur within 48 hours, so you cannot be pregnant from the 11 march.

The only thing that works is that the scan date is out by the 3day error of measurement, and so you actually ovulated on the 9th March and that the sperm caught the egg on the way down the fallopian tube. If this was the case then you could argue the case that your due date is 30 nov.

Having said all that, the health professionals will take the scan date as it is out by more than a week. (They did this with my daughter, but she arrived even earlier than the revised due date!!!!)

The important thing is that you have a wonderful healthy pregnancy and that you know that hopefully, if the baby goes to term, that you will give birth sometime around the end of November or beginning of December!

lilstar Wed 22-May-13 16:38:40

Thanks for your replies. I realise I mentioned about my period being on 27th Feb. The reason I was questioning that part is because since finding out I am pregnant and tryng to work the actual date I got pregnant I have read about implatation bleeding which I never knew I exisited before! I already have one child and I never experienced implantation tht time. My periods have always only been for a couple of days heavy bleeding then virtually nothing. On the 27th Feb I did have a couple days normal heavy period like bleeding and wondered if this could be implantation and that I was pregnant earlier than I thought.

willitbe Wed 22-May-13 18:29:05

If you were pregnant from the LMP of 30th Jan, and the DTD on the 14th Feb, with implantation bleeding on the 27th Feb, this would put your due date as 6th November and mean that your scan yesterday would show you as 16 weeks pregnant.

Clearly this was not the case as the scan would not be 4 weeks out, it could have been a few days out but not weeks at this stage.

So your last AF was indeed the 27th Feb, and you conceived from the DTD on the 11th March. Implantation bleeding is not very common (seems more common reading MN!!!).

I think you can safely assume a due date around the end of November to be accurate!!!! HTH

lilstar Wed 22-May-13 21:59:32

Thanks alot willitbe, what you have said akes things alot clearer and certainly has helped.

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