Dating scan and conception dates don’t match

[Soakitup37]

I know when I conceived as it was the only time I’d had sex in about 2 months, but when I went for my 12 week scan they put my edd forward by 5 days, meaning using my edd - my conception date effectively happened before I’d had sex!

I’ve heard of dates being put back because of later ovulation etc but never forward, has this happened to anyone else before?

I thought edd from scan dates were quite accurate, could it be out by 5 days? My concern is when it comes to measuring the baby as pregnancy progresses or when I reach 40 weeks and start being “overdue” I won’t actually be overdue by my own calculations, which I’m worried will force me to be possibly induced before Baby is ready.

Yes this happened to me. Conception can of course, be up to several days after you have sex, but obviously not before it as you state!

I actually knew when I ovulated as I track cycles with BBT (as well as using OPKs). I'd already had a scan at 11 weeks when I went for NIPT screening and that scan agreed with my dates to within one day (which given there is measurement error seemed reasonable) but that only used CRL (crown rump length). Dating scan 2 weeks later put me forward by an entire week, assuming immaculate conception... But they included head circumference in that one and my OH has a huge head so we were pretty sure at that point that that was what was throwing it out. Baby was ultimately born with a head over the 99th centile and is still over 99th centile at 2 years old.

The dates off scan measurements go off the 50th centile. It's a bell curve, so although a large group of people won't be that far off that measurement, clearly not everyone is going to fit close to that. I find it a annoying that they assume this if a woman does actually know more accurate information, e.g. had confirmed ovulation or IVF transfer etc.

I argued my case with them for similar reasons to what you state and managed to get the dates corrected by about the time I got to the 24 week appointment. I was glad I did as ended up needing an ELCS (PPROM and breech) at 37 weeks and they'd have scheduled it for a week earlier when the baby was only 36 weeks had I not got that corrected.

Yep, by more than that. Baby arrived on the day I should have been 40 weeks. Happens quite a lot as they're only tiny so measurements can be out, just get it on record with your midwife and push back if they try to induce based on the wrong dates.

I ovulate on day 20/21 instead of 14 which is ppl's normal. I am currently projected to be due on August 30 by the dr based on last period date, but I am actually sure my EDD is early September based on the ovulation date which i know is right. Either way, baby will come when he comes I guess :)

Happened to me both times, with my first DD they put me ahead by five days, and she arrived on what would have been her due date had I gone by the date of conception and not the scan.

Pregnant again and this time have been put just 2/3 days ahead of conception date, we only DTD the deed twice that cycle and on consecutive days.

They date at your 12 week scan based on average size. So it won't necessarily match up with conception - some babies grow quicker/slower than others. This happens to people who've had IVF who know the date without any possible doubt. It's nothing to worry about.

Yes. Both my babies were put forward 7 days by edd. Both were born fine and my first was born at 37 weeks by their dates at 8lb 4! (34 weeks after we dtd)

Some people just implant early/grow them fast

Yes this is super normal! My due date was adjusted to be 5 days earlier than from LMP and as I'd been tracking ovulation and tested positive at 8dpo I know it's not quite possible. The dates are always estimates though, the duration of a normal pregnancy varies naturally from 37 to 42 weeks (so when natural labour starts when baby is ready), and only about 4% of babies are born on their due date. So 5 days out is really not a big deal.

This is really reassuring and interesting to read, thank you for your comments!

Had the same and was very confused as I knew the exact date!

Yes this is happening with me too. I should have been 12+2 on my scan and they put me in the 13th week: Iv had a few early scans for bleeding and baby has always jumped a few days ahead each time. So now going back I apparently basically conceived at the end of my period lol. I know I didn’t I know the day I ovulated but it’s all fine I’d rather baby was thriving and growing a bit faster than not growing fast enough and there being concerns

A scan is as only as accurate as the scanner....please don't worry. Its just used as a rough guide for medical and midwifery purposes. I had the exact same as you in first pregnancy.... I had a private early scan by a very experienced obstetrician and she gave me the exact correct date ( I knew the date as I had sex once in 8 weeks) but when I went for my nhs dating scan further along she gave me a different date 5 days out. She was a trainee sonographer. I wouldn't get worried about it op. Like I say it doesn't need to be exact it's used as a guide xx

Lol I should add I used to be a midwife and I remember more than once seeing couples who had ivf so knew the exact date of implantation and they would have edd dates that were days out!!!!

I've never understood the UK obsession with exactly dating something that doesn't have an exact date! In France they give you a two week window rather than an exact due date - far more relaxing for all involved (& a lack of stress improves outcomes) and actually more scientifically accurate.

@1stWorldProblems I agree. It's one thing if the woman is unsure of dates, but if it's IVF or ovulation is known then it's just using a more unreliable method to calculate EDD 🤷

Hi and thank you for this thread. I learned a lot. I have a question that I am asking from a male perspective. Here's the situation: two weeks ago, I learned that my girlfriend was pregnant. The scan happened last week (July 26th) and they said that my girlfriend was 6.5 weeks pregnant, placing conception date on June 14th.
Problem is because she was travelling, I didn't see her from May 3rd to June 29th. On both these dates, we had sex. The scan operators said that fetal size can be influenced by partner size (I am 6'5).
In your opinion, is it possible to be that far off? Appreciate your answer(s)

It sounds like you don’t trust your partner…

They start to count the 40 weeks of pregnancy starts from date of last menstrual period, not the actual date of conception.

Ovulation (and so conception) usually occurs around 2 weeks after that.

The start of my pregnancy is being counted from 28/10/21, but I estimated we conceived from intercourse on 8-10th November, and I estimate that I ovulated some time 11/12th November.

So when I realised I was pregnant in December, I thought I was only 5weeks pregnant, but was told I was 7 weeks pregnant.

That’s not how it works, they date from first day of last period so even though the scan says 6.5 weeks, she foetus is actually around 4.5 weeks old, so she conceived around the 28th/29th/30th June.

sorry the above is meant for @Italo though I was replying to his message
it’s annoying that you can’t edit or delete posts!

@Italo pregnancies are dated from the last menstrual period, not conception, so a pregnancy that’s 6.5 weeks along is actually roughly 4.5 weeks from conception assuming ovulation around cycle day 14. So the first two weeks of a pregnancy the woman really isn’t pregnant at all!

Hi @Italo they add on 2 weeks to take into account a woman’s full cycle. So day 1 would be the 1st day of her last period. So conception would be roughly 2 weeks after this. So working backwards from the dates you gave actual conception would have occurred roughly 4+5 weeks ago from the scan date which tallies up with when you have intercourse on the 29th June. That is very likely the day you conceived. So congratulations daddy to be 🎉🎉 wishing yr GF happy smooth pregnancy xx

@Sunnydayz The element I didn't mention is while she was travelling she told me her ex flew to the same destination as her, which coincides with the said date of conception. In my past, I've been in 4 serious relationships and been cheated on 3 times. So even if she shared nothing happened, I have my issues with trust and I don't deny that.

@Sunnydayz @Hatscats @GinnyBee @EmzieMurf1987 I appreciate you all taking the time to provide me with clear explanations and for the congratulations messages. Apologies if I sounded rough or rude at any point, that wasn't my intention 😊

@Italo sorry to suggest that you didn’t trust her, I regretted saying that as soon as posted.

I just know I would be so upset if my partner had any doubts about the baby not being his, but I can understand if the dates don’t match up you would begin to wonder!

Can I share more of my story? My partner went on a surfing trip with the lads on the 11th Nov (I calculated we conceived from sex on 8-10th Nov) and I went surfing with a friend / old hook up on 14th and 15th November, we drove a couple hours to the next county and back again, didn’t stay overnight anywhere together, but did spend 2 days together, driving, surfing, getting food. Nothing remotely romantic happened, we just hung out, caught up and went for a surf.

If something HAD happened, (in addition to the relationship being f*cked) there would be doubt about the father of this baby, I would not have felt good or excited about the pregnancy. I think I would have felt sick, and would have considered not telling my partner and terminating rather than dealing with the mental anguish of being unsure about who the father was… not sure if that helps at all! Just my experience…

I get damage from previous relationships can be hard to shake, but it’s not healthy to hold on to it.

I do believe congratulations is in order, so congratulations to you both 😀

The first trimester is super tough! And a women gives up a lot to carry a child. Be kind and supportive ❤️

@Italo
oh and I still had doubts and moments where I cried and regretted being so irresponsible…(it was unplanned and we hadn’t been together long) but now I am so happy and excited for the arrival of our little one

@Sunnydayz, nothing to regret, you were absolutely spot on. You are absolutely right with the fact that holding on past grudges is unhealthy, at best. And I am dealing with that currently, as well as some other insecurities.

Everything you mentioned has happened or is happening: she's bewildered I can even doubt about paternity. But nobody provided me this 2 week-explanation before. Despite having done research, I just found this forum and you guys today.

She has explained the same rationale you have, re-doubt of paternity: she would have felt sick and wouldn't have 'entrapped' me to raise another man's child.

I have tried being supportive and be there every step of the way. Because of my initial doubt and also personal circumstances (earning decently but very worried because I have a student loan to pay off for the next two years - meaning less disposable income), she just told me she doesn't feel secure enough to go forward with this pregnancy. My anxieties took the best out of me and I have ruined something great.

Thank you so much for sharing your story. I know see how 'dumb' I might have sounded and how hurt she is.

@Italo
Sorry to hear about the outcome. You both sound young, and at least you know you are fertile so children later down the line will be possible.
Perhaps now isn’t the right time for either of you, or the situation or the relationship is not right. Don’t beat yourself up too much. Everything in a life is a lesson.