38 weeks or 40?

They calculate the weeks from the 1st day of your last period, if you know the date of your last period, generally they generally say add 11-21 days for date of conception.

For example if you had a period 1st-5th March, your conception date would be roughly between the 16th-21st March.

So technically we’re only pregnant for around 38 weeks but they count from the last period so that’s where we get the 40 weeks from.

Ah okay. I never had a period before hand due to taking contraception not carefully so i’m not sure how i’d work mine out

At your 12 week scan they can date the pregnancy and they give you a due date, which is what the nhs uses.

So at your 12 week scan do you work it out as 12 weeks before to give you a 40 week pregnancy or 10 weeks for 38?

First day of my last period was 5th march, I had 10th December as estimated due date when I went for my 12 week scan and did measurements I was given the 12th December as due date which NHS will only use. I would not worry too much about working out your dates because once you’ve been seen they will tell you ☺️ x

You don’t work it out at the 12 week scan, the obstetrician or sonographer will use the measurements from the scan to give you a due date. Before the 12 week scan you can work it out as 40 weeks from the first day of your last period. On the nhs website there is a due date calculator that you can enter the first day of your last period and it gives you your due date but the most accurate date is from your 12 week scan. Due dates are a myth though - your baby can be born anytime between 37-42 weeks.