If you're 1000% sure of your last period...

[Opheliasgoldenwine]

And your scan contradicts it, then how does that work?
This happened to my friend (2nd baby) who knows when she last had her period but the scan she recently had said she was a week earlier than what she thought. I'm only asking as I have my scan next week and I categorically know when my last period was (religiously track it) but how does it work if the scan contradicts? Is it just late ovulation? TIA 

Happens all the time. They’ll go on the size baby is measuring, because a 40 week pregnancy is only an average anyway

Dating a pregnancy from last period is based on a 28 day cycle with ovulation on day 14 so if your cycle is different for this your date from scam will be different.
My scan due date fitted when I ovulated better than my dates by last period.

Because your last period doesn't tell you when you ovulated.

So if I have a 25 day cycle and I ovulate and conceive on day 9 and my friend has a 35 day cycle and ovulates and conceives on day 23, we have the same date for our last menstrual period. However, my pregnancy will be 2 weeks ahead of hers.

Also, time to implant varies from woman to woman so last menstrual period is just a rough guess until the scan gives a clearer picture.

To be that sure, you’d have to know exactly how many days elapsed between your ovulsion and period (Its not always 14 days.)

I knew when I ovulated and they still didn’t change my due date which then affected indication date.

This happened to me - my scan told me I was due 8 days before i knew was possible based on both my period and when we had sex (baby no 2 so we were only managing once a month at that point!). My midwife wasn't prepared to go by my date but I just kept repeating it firmly whenever it came up and making it very clear that I was going to go by my date. In the end ds2 arrived 13 days after the official date (so 5 days after mine), at home as i planned, but I had to resist quite a lot of pressure to go for scans, sweeps etc when it got to that stage. If your friend is happy with her date and remains firm they can't actually make her comply with interventions and so on, but it can be stressful if her midwife isn't supportive (although i believe many would be).

Sorry read too fast, it's still hypothetical for you - I wouldn't worry at this stage.

Oh and if you know when you ovulated just adjust the period date if not accurate. I ovulated a week late (day 20) and just told them my period started a week later than it did. But ultimately don’t worry too much, babies are born from week 38-42 so the exact dates don’t really matter r

You can get spotting when the embryo implants in the womb a few weeks in. With both my kids, I mistook this for a period and it turned out I was further along than I'd thought.

Even when you know for certain when your period, ovulation, and implantion was, the dating scan is just a measurement that determines the size of the baby and that size then becomes a date based on the average age of a baby that has the same measurement.

Dating scans rarely match up exactly even when it’s an IVF pregnancy and there is little left to question.

As PP say, your last period date doesn't tell you when you ovulated. Or conceived for that matter considering spern can live in your body for a number of days. An early scan is the most reliable method of calculating a due date but only 5% of babies come on their due date anyway so it really doesn't matter.

Both my ivf babies (so really cannot question dates) I had my due date changed at my dating scan! They always go by the scan.

Same as @pigsknickers (love that book) Was so sure of ovulation date that I delayed breaking waters and induction as long as I could because they had DS five days ahead of schedule.

I’ve known exactly when my last period was in all 3 pregnancies but dates have still changed with my scan. Last period is only a very tough guide, and used because all women have different length cycles/ovulate at different times etc. As a general rule the scan date is far more accurate.

And if you think about it, of course they can't take a woman's word for it because of human error or deception, many women each year who state they definitely conceived on x date will be wrong. Whether because she's deliberately misleading staff or misunderstood biology or simply written it down wrong, taking her word for it would directly affect the care of the baby throughout pregnancy. We have to use the scan dates. Of course, occasionally this means that you might have to be more insistent about expectant management at the end but mostly, the scans are pretty accurate.

I ovulated 4 weeks after my last period due to long irregular cycles so I was only 11+3 at my supposed 12+6 scan.

I have a long cycle and despite always telling GPs and midwives this , my due date was always calculated based on my lmp and a 28 day cycle. Consequently all my babies were quite 'late' which did mean I had to be quite firm about resisting inductions and sweeps.

WildCherryBlossom, did you not have scans?

If you go by my scan date I'm currently 11 days over due. Go by dates and I'm one day over! They won't listen to me though and are sticking with my scan dates. I know my dates are right!

Yeah I see what you mean but in my case, I’d had an internal scan 2 days after ovulation which confirmed it so I was a bit miffed.

Confirmed that I’d just ovulated, obviously not confirmed the pregnancy!

If you ovulate early or late.

Showofhands yes of course - but due dates were always calculated by my lmp and assumed a 28 day cycle.

But that’s why they change the dates at the scan when they’ve measured the baby. The know not everyone has a 28 day cycle but there is no point worrying about it when you can use the scan measurements

As Capri says, that's normal. The lmp is used to calculate due date until your scan where they adjust if necessary.