Help me understand dates!

[JoCF42]

Okay so first day of last menstrual period 14th November 2019. We started trying from the 17th November and then I found out I was pregnant on the 10th December from an early test. I have a normal cycle averaging at 28 days.

I had two early scans at 7 and 9 weeks and the sonographer (private) said my dates were spot on and put the baby's due date at 19th August.

Just had my 12 week NHS scan and have been told I am definitely 13+0 weeks pregnant (I thought I was 12+2) and that due date will be on the 14th August. I know this is only a 5 day difference but how is this physically possible if we only started trying on the 17th November? We were using condoms and have never had a mistake before and weren't even having sex the couple of weeks before trying as far as I remember!

Very confused...

It is confusing.

You count pregnancy from your last period, despite it being about two weeks before you could possibly have gotten pregnant.

Most people don't track their ovulation and they certainly didn't in the old days, so you just go by date of last period. So you can be 13 weeks pregnant but the baby was conceived 11 weeks ago.

Does that help?

@corduroyal Thank you for your response. So does this mean I probably just ovulated earlier and can still be 13 weeks?

I'm no expert but having had similar confusion over my own dates, I was told that the first day of your LMP is just an initial guide and is based on ovulation occurring exactly 14 days after that. Most women will be slightly more or less than the 14 days though so the actual due date will be different. Maybe you just had a short cycle that month?
At the dating scan, they measure the CRL of the baby to get the age and due date that way and it's apparently more accurate than going solely from the LMP.

Also, it's harder to get an accurate age at an early scan as the baby is so tiny. Hence why they wait till 12 ish weeks to give you an official due date

To be honest, I don't know OP.

It's all a bit of a messy process really - I personally also think they should replace due dates with month-long windows, as babies are considered full term by 38 weeks but it's not usually problematic for them to hang on until 42 weeks.

Just to confuse you more! Congrats on the pregancy, headline is that the baby will get bigger until it's ready to come out

@JoCF42 - you have just posted my exact situation - right down to the dates. I equally have no clue how its possible.

though more frustratingly by putting me a week ahead, it now means I need a fit to fly cert for our holiday which I thought I had avoided.

@ml01omm it literally makes no sense - how frustrating about your flight! Hopefully the cert will be easy to get sorted