Due date confusion - is it possible?

[FedUpOfLighteningCrotch]

I’m due 26th March - that’s the date given by NHS 12 weeks scan. It was earlier based on my LMP before my first scan.

Anyway I’ve never really thought much into it until I’ve been speaking with a girl who has a very similar cycle to me and I noticed her due date is a lot earlier than mine! 14th March.

So I looked back at my cycle tracking app and checked when I had my positive ovulation test which was 1st June, so I would have ovulated 2nd/3rd June - 40 weeks from then would make 10th March I believe?!

I also read on a forum (I think it was on mumsnet somewhere) that the user had told her midwife when she had ovulated they went by that date rather than the scan date - so..

Should I have told my midwife when I ovulated; would that mean my due date is more accurate? My last baby was a small 4lb baby so now I’m worried that the 12 week scan showed a later due date because I just grow small babies 🤣 is that possible or am I being mental? 🙈

I’m going to be on edge for the entire of March now!

Watching this! I get different due dates on websites depending on my ovulation date and the first date of my last period. I know both dates and my exact ovulation. I either get 6th October or 10th. So strange! I haven't had my ultra scan yet so would be interested in how they work it.

In terms of baby size, all babies grow at about the same rate up until 10/12 weeks, and it's only after that that genetic differences set in (so two 20 week babies might not be the same size, although they were the same size at 12 weeks). This is why the NHS is able to use the 12 week scan to date the pregnancy, but wouldn't re date it based on baby size at a later date.

Hi, how confusing!

I don't knw enough to address a lot of this. Just to say, I knew my LMP and when I ovulated (I have a long cycle and ovulate around day 30). Anyway despite telling my midwife this she worked on LMP to schedule my 12 week scan (which was then too early due to late ovulation!). At the second 12 week scan I got the new EDD and that was then gospel.

So my experience is even if you tell them your ovulation you'll be ignored and they'll go off the 12 week scan. I thought the margin for error with the dating scan was about 3 days either way so this doesn't really explain the discrepancy between the 14th and 26th.

Having said that, my LMP was 31 May, and my due date is 6 March, so something does seem slightly out if your due date is the end of March but your LMP was (presumably) mid May. The calculation is about 40 weeks from LMP I believe, not 40 weeks from conception.

I would always go off my ovulation date, if I knew it. All health care professionals will go of the scan though. I got 3 different dates from my 12 week scan as it all depends on baby position and the sonographer measuring. They picked the middle one.
As they have dated you later, you won’t at least get the pressure of induction for post dates!

Hi @FedUpOfLighteningCrotch - the 40 weeks (or 280 days) is added from the first day of your last period. It is calculated on the assumption that you ovulated 14 days after that based on the average 28 day cycle with an average ovulation half way through.

If you want to work out your due date based on ovulation date, you add 266 days from the date of ovulation (280 - 14 = 266).

So if I'm doing my maths right, if you did ovulate on 2nd/3rd June, your due date would actually be 23rd/24th February!

However, a positive OPK doesn't mean you're necessarily ovulated straight afterwards. The test can also sometimes detect your body trying to ovulate - detecting an LH surge - but then your body doesn't actually ovulate and you would then get another peak later.

It is very unlikely that your dating scan could have been that far out if you did the scan around 12 weeks. A dating scan at 12 weeks can be 2 or 3 days out, but not weeks out. It seems much more likely that you ovulated later than you thought?

When was the date of your last period and how long are your usual cycles? Are they regular?

@cunningplan101 oh how bizarre!

The first day of my last period was 12th May, and I’d recorded having sex on the 1st, 2nd, 3rd and 5th of June (not a very high libido so sex only really ever happens on special occasions or when ttc around suspected ovulation) my cycles are usually anywhere from 28-35 so perhaps that might explain some of it as they’re not that regular but not few and far between either!

I think we’ll just have to wait and see if I go overdue the 26th! 🙈

When did you get a positive? I ask because your due date is one day before mine. I have an LMP of 14th June (32-35 day cycle). Unknown ovulation date, but I'm reasonably certain I conceived in the first couple of days of July. I got my BFP 16th July.

Yes it would be useful to know your BFP test date.

If you had sex on 5th June, you could have ovulated on 10th June and still become pregnant. But that would still put your due date on 3rd March.

A due date of the 26th March would suggest an ovulation date of approx 3rd July. So you would needed to have had sex approx in the five days before that (as sperm can live for up to five days)

When you went for your scan, did they adjust your due date by six weeks?

(Also, did you have a Covid jab around those dates by any chance? That has been known to delay ovulation or make some women skip a whole period)

@Dyra @cunningplan101 I got my first positive on 12th July and my period was ‘predicted’ to come on the 16th but obviously that’s always give or take a few days. I must have had a really weird cycle that month - and thinking about it when I got pregnant with my first child I didn’t do much ‘planning’ when ttc so I didn’t track anything other than my usual period prediction day to get an idea of when I could test.. but I remember thinking with that pregnancy that my due date was weird compared to my last period.. but I didn’t have the foggiest then of when I ovulated anyway so probably means nothing 😂

No, I hadn’t had the covid jab. I decided to wait until after the 12 weeks scan to have my first jab as I didn’t want the jab to delay my periods/mess with my cycle while ttc 🙈

I think your scan dates are probably correct then. Certainly lining up nicely with your BFP anyway. Did you do any pregnancy tests when your June period was due? Or ovulation tests late June/early July?

I didn’t have a period since May 12th, positive OPK on 1st June.

I definitely don’t feel like baby is going to arrive anytime soon, albeit being so achy and fed up!🤣