Measuring ahead on dating scan?

[BabyWA]

Hi! Just looking for some advice... I'm currently in my third trimester and I have not really questioned this until now. My LMP was 4th July and I have 30 day cycles-ish, well I'm regular anyway. I wasn't tracking ovulation but looking back at my dating scan on 25th September, they dated me 12+4 with a due date of 5th April? Meaning that my LMP started 29th June.

I'm not too sure how it works, but I thought my due date would be around 12th April! I'm a bit worried if it comes to being induced or scheduled C-section and my dates are wrong. According to my scans I would have ovulated on cycle day 10?! Is this even possible?! 😆

I'm just wondering if anyone else has had this?

Your dating scan would have been based on the size the baby was measuring. Usually if it just out by one or even 2 days they won't change your due date but if it's out by 5+ days they'll redate you.

Yes you can ovulate on CD10. If you ovulate as early as this, you are less likely to get pregnant but it is possible.

I ovulate CD12 and get pregnant easily. If you ovulated CD12, your dating scan is only 2 days out, which is perfectly normal. I track ovulation with two pretty precise methods, and have been dated 3 days out each time. I just remembered I had that extra 3 days and would have refused induction (or requested it 'early', depending on which direction they were wrong)!

It’s only a few days difference, I really wouldn’t worry. It’s all an estimate anyway. If you don’t want an induction or c section then just say no or that you want to wait. It’s normal to go to 42 weeks unless there’s a genuine concern. You can definitely ovulate on day 10.

This is exactly the LMP vs scan dating confusion so many people run into! From what I understand, dating scans (especially done in first trimester like yours at 25th September) are actually considered more accurate than LMP-based dating, particularly if your cycles aren't perfectly textbook 28 days even if they're regular. A 30 day cycle can shift ovulation later than the "standard" calculation assumes, which would explain the week-and-a-bit gap you're seeing.

First trimester scans (done between about 8-13 weeks) are accurate to within 3-7 days, which is honestly more reliable than period-based dating for most people. So if there's a discrepancy, doctors generally go with the scan date, not the calendar calculation.

For induction/C-section scheduling specifically, they'll almost always use whichever date was established by that early scan rather than recalculating from LMP, so you shouldn't need to worry about your dates being "wrong"