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Conception date confusion.

(20 Posts)
Cazzr Mon 15-Nov-10 11:06:28

Ok, so I had my '12 week dating scan' last wednesday and they measured that I am a whole 9 days further along than by the whole LMP calculation....

Now, I thought I might have ovulated a couple of days early but 9 days... equally, I doubt we ttc around that time as (tmi warning) I'm pretty sure I was on last dregs of period still at CD6.

So my question. Is the conception date when you ovulated or when the egg and sperm met?
I could understand if it was from ovulation but not if they think we dtd that early...

Now it's not a big problem right now, but as I'm wanting a home birth, 9 days earlier could mean extra pressure to go for induction if I go 'overdue'...
confused

Wigeon Mon 15-Nov-10 13:50:51

Conception must be from when egg and sperm met, rather than ovulation. Eggs live for about 24 hours after ovulation, but sperm can live for days and days (can't remember exactly how long, but it's a surprisingly long time!). So you could have TTC on a Monday, but not ovulated until Saturday and still got pregnant. And you wouldn't necessarily ovulate at a predictable time in your cycle - it might well have been "early" or "late". How regular is your cycle?

I see your point about the homebirth / induction...pretty sure that the dating scans are really quite accurate though.

TRL Mon 15-Nov-10 13:54:14

Hi, I had this with one of mine - the dating scan suggested I was due 9 days earlier than I thought I would be. Sure enough, DS2 arrived one day after they reckoned he would from his measurements at the 12 week scan....

Deliaskis Mon 15-Nov-10 14:06:12

Hi, we had the opposite, conceived (slightly by accident blush) very late, in fact the reason for the accident was that I had assumed it was too late to conceive so didn't bother with contraception. I was expecting my period about 3 days after.

So my LMP date was completely inaccurate, and by 12 week (which was closer to 10 weeks), 13 week nuchal, and 20 week anomaly scans have all agreed on my revised due date which is 9 days later than my LMP date.

My cycle is v regular, I can only assume that I must ovulate late in the month (had never tracked as weren't TTC!). Obviously until the baby arrives, I won't know any more, but just thought I'd add my experience so far as LMP date was completely inaccurate and resulted in an assumed conception date which was impossible.

D

BayeauxT Mon 15-Nov-10 14:08:02

I could have written your post Cazzr - my due date by lmp is 2 May but by scan it is 23 April (in fact I was one day away from being too late for nuchal test!) I've always had a regular 28 day cycle, so it seems strange that this time I was off... we weren't actually trying and I know we didn't bd a couple of days either side of expected ov... Like you I am also hoping for a home birth so I am a bit worried about this!

Cyclebump Mon 15-Nov-10 14:15:21

I thought they dated it from when your last period ended... Clearblue digital thingimagig said in its packaging that if the test came up 2 weeks since conception you would be described as 4 weeks pg.

Cazzr Mon 15-Nov-10 14:15:22

Thanks guys but the dates they've given mean we conceived on CD6 and i KNOW we weren't doing the deed at that point, earliest was about 3/4 days later assuming conception happened pretty much straight away..

My cycles were 29 days on the dot and I was temping, charting CM and ov stick testing and think I ov'ed about cd10 at earliest, tho previously ovulated cd15 every month. Typically that month things were less clear..

I guess they do say the date can be 3/4 days out so I'm thinking they are out by that much..

Incidently, can a big baby measure further along at this stage? I'm 6ft 2, DH 6 ft 1 and DS is, and was at birth, pretty tall...

FYI DS was born after pessary induction at 42 weeks so aware going overdue is a possibility.

Showaddywaddy Mon 15-Nov-10 14:19:45

At this stage development is pretty uniform. Hence the dating scan now. Genetics will start to make a difference in the 2nd trimester.

It's probable that it's 3 or 4 days out as it can be and you ovulated on day 9 or so.

Cazzr Mon 15-Nov-10 14:20:33

BayeauxT, sorry cross posted. This change has bought me from 24th May to 15th May (ironically DS was due 14th May).

It might be a non issue and plan to discuss with the midwife (who is pro-homebirth) but wanted to clarify meaning of conception date.

Showaddywaddy Mon 15-Nov-10 14:20:49

Cyclebump, pregnancy is dated from 14 days before conception. The theory is that the average woman will begin her period around 14 days before ovulation. But there's a massive amount of variation.

Cyclebump Mon 15-Nov-10 14:29:39

Ahhhhhhhh, all I know is it was bang on when it came to me, despite the mw and GP being sceptical about 'shop-bought nonsense'.

JBrd Mon 15-Nov-10 14:37:38

I found as well that the dates the midwife/scan suggested seemed more than a week off from what I had been calculating. But then I would always take my actual assumed date of conception as the starting point, whereas they'd calculate if from the first day of the last period etc.

What I don't understand is the whole '+ x' days thing - dating scan notes say I was 13w +2, for example, how does that get calculated?

Cazzr Mon 15-Nov-10 14:42:23

JBrd: 13 weeks and x days.

BabyValentine Mon 15-Nov-10 14:43:53

Cazzr, I had this same problem. I was charting and using ov. sticks, so am pretty certain when I ovulated. I also know when DH and I had sex grin.

My dating scan has put me 6 days ahead of my dates, and I know for a fact that we hadn't yet had sex that cycle. So I am assuming that I might be a day out (although it was a classic triphasic chart) and the scan might be a couple of days out, and we'll meet in the middle somewhere.

Hoping DC isn't late, so I don't have to have the induction conversation. FWIW, although the sonographer poo-pooed my ability to know when I ovulated, my MW was fairly encouraging and said I was probably right (although they have to use the dating scan figures).

Wigeon Mon 15-Nov-10 18:54:31

Would the fact that you won't necessarily conceive on the day you have sex (because of eggs living 24 hrs and sperm living for days and days) help explain some of this?

You can't assume that if you had sex on the Monday you would conceive on the Monday. Actual conception might happen on Wednesday for example, with sperm still wiggling around from the Monday.

Obviously you wouldn't have conceived before you have sex, but you might the day you do the deed, or several days afterwards.

Cazzr Tue 16-Nov-10 09:33:49

Wigeon, this is sorta my point. I was at the tail end of my period on CD6 and hadn't had sex on that day let alone whilst I was on my period so I simply can't see how it's possible unless they are 3/4 days out (my assumption now) and I conceived the same night we DTD... I'm pretty sure we DTD on CD10 and not before you see...

The irony is that we had pretty much written that month off because we had a LOT going on and were quite stressed. hmm

BayeauxT Tue 16-Nov-10 09:45:52

I think you can conceive the same night you DTD - googling it, if your DP has fast swimmers it can happen in a matter of hours.

A friend said her midwife told her the scans can be a week out - a week!!! Which is not what I wanted to hear. I will raise it at my next scan.

Cazzr Tue 16-Nov-10 10:57:56

Heh, sonographer would only admit to 3/4 days tho...

piprabbit Tue 16-Nov-10 11:02:42

I had IVF for DC2, so I knew exactly what day the eggs were collected and fertilised. Absolutely no doubt.

The hospital scan date came up with a due date that suggested the baby had be conceived 8 days earlier. They insisted that they were right and kept their dodgy date on my records. They kindly {hmm} let me go a week or so overdue (according to them) and DS was actually born on his (IVF clinic) due date.

Try not to worry, and don't take the scan dates as gospel.

cowboylover Tue 16-Nov-10 18:54:31

Good point piprabbit: My consultant is very nice and really real if that makes sence! He said no dates are accurate and they can not make you do anything you dont want to so if you are not ready to be induced make sure they know it and you can refuse.

All the dates vary so much and I have scans every 3 weeks due to complications and he comes up with a different week and day! I am going with 15th of May for my official due date (this week!) so I keep saying the middle of May when people ask!

I hope it all goes well for you and for the homebirth smile

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