How do baby measurements work on ultrasound

[Slkx]

Hi, would anyone be able to help me please I am currently 13 weeks pregnant, I went for an early scan when I was 8 weeks I asked the lady for my conception date and she told me it was from the end of August. My pregnancy is being measured from 2nd September my due date is 9/6/25, when I read online it says I would of conceived middle of September don’t know how that is possible because apparently the first two weeks you’re not actually pregnant but it’s added into scan measurements does anyone know if this is true? Am I only technically 11 weeks pregnant not 13?

Have you had your NHS scan yet? They'll use the size of the baby at that scan to determine how pregnant you are and what your due date is, regardless of your LMP or knowing when you ovulated or anything. Nothing overrules that.

Yes I had my nhs scan, and got my due date. Just when I was told that what baby measures is actually 2 weeks less as they add two weeks in that you’re not actually pregnant 🤦🏼‍♀️ just confused me. Thanks for you’re reply

If you took a supposedly typical cycle of 28 days, ovulation occurs on day 14; the egg will have to be fertilised within 24 hrs of ovulation. Implantation is then around 6-12 days later.

As such.. depends when you feel you technically are pregnant - when the egg is fertilised or when it implants.. but because of all that variability we calculate pregnancy from the first day of your last period.

That does mean that (on average) you're not pregnant for the first 2 weeks (or longer if you're waiting for implantation or when it can be picked up on a pregnancy test). But.. this will vary from person to person.

At a dating scan they may adjust your due date based on the size to account for this variation but that's still not an exact science.

Thankyou for your reply, so when they measure you on a dating scan the size the baby measures that how long ago the baby implanted? That’s what I always thought, but then someone told me that’s incorrect as they measure the baby on dating scan but there is two weeks added in so baby measures say 13 weeks but really you’re only a 11 weeks pregnant? It’s confusing to me. Yes my periods were very irregular and I was on conception so I wasn’t measured of a LMP

My nhs 12 week scan measured up to exactly when my last period started. So my last period was 23rd December (in 2923) and at my nhs 12 week scan it actually put me at day before my period started somehow don't ask lol. I went in thinking I was 12w4 from the first day of my last period but my baby measured 12w6
I think technically you don't count the first 2 weeks at the start but as the baby grows it matches up with your original dates I think anyway
At your 12 week scan is it far off from the date from your last period?

2023** 🙈

The standard way of measuring pregnancy is from the first day of the last period, as most people don't know when they ovulated. This does mean that technically you're not pregnant for the first two weeks but there's no sense getting hung up on that because all milestones appointments etc are based on the weeks since lmp, or when your lmp would have been if you had regular periods.

My dates are very similar to yours. First day of my last period was 10/9, I ovulated around the 25th and I'm currently 12 weeks 2 days pregnant which my dating scan didn't adjust.

No mine didn’t measure up on my 12 week scan, my periods aren’t regular at all, I’m measured from the 2/09 on that day I did have a one day bleed maybe that was implantation 🤷‍♀️.. I asked the ultrasound lady about the two weeks and she said not to pay attention to it but it’s all I see online just made me think how they measure a baby to an amount of weeks but two weeks the baby wasn’t actually there yet lol, I was just curious to know if I’m actually 13 weeks or really only 11 but measure 13

Whatever they told you add the appointment will be the number of weeks - or work back from your EDD if you prefer. You're 40 weeks at EDD

Your baby is measuring 13 weeks that's the main thing. It doesn't matter about the 2 weeks now, it's how big the baby is if that makes sense?
I went for a private scan at 7w2 but she estimated me at under 6 because all she could see was a sac but by the time I got to my 12 week scan my baby caught up to exact weeks of my last menstrual period

Yes it includes the two weeks before ovulation in the total number of weeks. So technically not pregnant for the first two weeks. I’m now 20+4 according to the nhs and that include ovulation-2 weeks in that.

Pregnancy is measured as the life of the egg which then becomes your baby.

So the egg is released at the start of the cycle and, on average, you then ovulate 14 days after this (hence the extta 2 weeks that get added on). The day you actually ovulate and the egg is fertilised, you're technically already 2 weeks pregnant as the egg is 2 weeks old if that makes sense.

LMP is just the rough guide they use initially to book your appointments in etc but it's based on a 28 day cycle and isn't accurate for a lot of women (i'm 24 weeks and baby is 10 days behind what LMP would put me at because i have longer than average cycles).

The 12 week scan is the most accurate time to 'date' your pregnancy as its the clearest time to take measurements, it's very difficult to get accurate measurements before them as baby is so teeny and they're literally working in fractions of millimetres. The due date given at your 12 week scan is the right one.

To work out the approximate date when you actually conceived, you count back 38 weeks from this due date which for you means you conceived in mid September, you then add the 2 weeks on for the start of the cycle of the egg and that's what gives you the 40 weeks.

Pregnancy isn't counted from when you conceived, it's counted from the egg being released.

@sel2223 it’s been quite a while since I took biology but I read quite a bit while ttc. An egg isn’t released at the start of the cycle, an egg being released is ovulation which is ~2weeks into the cycle and then it’s fertilised within 12-24hrs after that otherwise the egg is too damaged. The fertilisation also occurs in the fallopian tube (thats what I read anyway). Implantation then takes 6-12 days (I think it said 6-9 is more typical but sources were varied). Then a positive test a few days after that if it’s really sensitive.

The start of the cycle is follicle hormones increasing to stimulate immature eggs to mature. A dominant one appears. (A bit hazy here- but at some point once the dominant one is ready LH is involved to say an egg is ready). Then the egg is released and that’s ovulation.

The timing of ovulation is different for people (no idea why- maybe varying levels of hormones I’m only guessing didn’t fully read into it). For instance I ovulate on day 18 and not day 14. So I tracked ovulation using lh tests and bbt. Then for my estimated lmp I just minus 14 days. To be honest I’m close to the average this time, with my son I ovulated on day 24 so it helped that I tracked to have an estimate lmp.

Actually ignore my comment @sel2223 I just focused on the word release but it makes sense about the egg life.

It's OK, I probably haven't used the right terminology there and I'm no biologist by any stretch but that's how i remember it being explained to me that it's basically the life span of the egg

Yes my LMP was on the 6th August and I have a 35 day cycle that month as I didn’t have a period in July I had two in June my periods were all over the place as I was on birth control, I have a day bleed which was on the 2nd September and that’s when they have dated my pregnancy since just no dates really match up to me 🤷‍♀️

Went I went for an ultrascan when I was around 7 weeks I asked the lady doing the ultrasound and she said the date of implantation was the 2nd September, I asked her about the two weeks but she said not to pay attention to that. But when I read online my due date would mean that I conceived middle of September, I also have a friend who is one day ahead of me in pregnancy and she didn’t have any sex in September so it really baffled me how we can be the same weeks in pregnancy but conceived different times

If you're "measuring 13 weeks" now then you conceived about 11 weeks ago, you're right they add 2 weeks.

@SapphireOpal is that even when a LMP is not known?

Yes. Your LMP is irrelevant for dating now you've been scanned, but they still add the 2 weeks so everyone's dated consistently.

@SapphireOpal oh okay Thankyou just thought I’d ask on here, as what I read online doesn’t make sense from what the ultrasound lady told me she said baby implanted 2nd September so I’m dating from them would of convinced around the end of August 🤷‍♀️

Yeah just forget about LMP now you've had your scan. That's the date to go with so if you're 13 weeks at the scan then you're 13 weeks pregnant.