To ask you to explain conception to me?

[AceOfSpades1]

You often hear of people saying their due date they calculated is different to the due date the are given at their 12 week scan or people believing the midwives have got their dates wrong.

I know people calculate their due date the based on the first day of their last period but I’d guess that date wouldn’t be all that accurate. I thought sperm could live for a number of days before actually fertilising the egg which would make it difficult to pinpoint exactly when the egg was fertilised? Surely a dating scan is more accurate than using the first day of your last period?

I think it's generally when people have been tracking their ovulation that they feel more confident about their dates (e.g. temping or using OPKs).

The only way to get an accurate due date is to use measurements on first trimester scans (all healthy babies grow at a uniform rate during this period). Periods, ovulation, the day you had sex are all unreliable because sperm can survive inside the uterus for several days, ovulation cycles can change unexpectedly, periods can continue during pregnancy etc.

But even if you know when you’ve ovulated, there is around a 6 day window isn’t there? So if the dating scan is say 6 days later than the date you calculated, would the dating scan be more reliable?

Dating scans are always more accurate, regardless of variables.

Dating scans are the most accurate, they're based on actual evidence of the size of the fetus.
Midwives with their stupid little wheels that assume a 28 day cycle are usually not accurate for most people.

The size of baby is the most reliable way to date and give correct due date. Midwives only use the last missed period as not everyone track ovulation etc.

My date was brought forward a few days after my 12 week scan and I know quite a few people this happened to as well - the measurements are the most accurate way of telling.

However I was a bit more surprised for me because I wasn’t TTC and knew exactly when my last period was and the one date on which I’d had sex following that - I wasn’t tracking ovulation so no idea when that might have happened, but obviously it can’t have been more than a few days after the sex took place.

(Furthermore, my due date has been and gone and still no baby...)

Our stupid little wheels aren’t just for calculating due dates. I find that at most due dates calculated by LMP are never more than a week out. But yes a dating scan is the most accurate way of calculating due date. However I have had women tell me their dating scan is definitely wrong.

'Our stupid little wheels aren’t just for calculating due dates. I find that at most due dates calculated by LMP are never more than a week out.'

Well mine was 9 days out. The MW didn't believe I have 19-21 day cycles. The scan confirmed my calculations were correct and the sonographer was annoyed because despite me having my initial booking appointment at 5 + 5 weeks my scan wasn't booked in until what turned out to be the day before I was 14 weeks which meant that I almost missed the window for screening. All because the MW relied solely on the little wheel, wouldn't listen and wasn't in a hurry to request a scan date for me*...

(* N.B. I blame the MW though, not the little wheel)

Both times I have been pregnant (currently preg with number 2), my due date has been out by 2 days when I have gone for the scan. I use a period app that I log my period into and it helps you work out when you ovulate. Must be quite accurate if both times I have been 2 days out

I absoutely knew when I had conceived with dc2, as I had sex and then my dh was away for 2 weeks, and we didn't have sex when he came back for about a week (cannot for the life of me remember why now)!

When I had all my scans, and when she was born, they all told me I must have the date of my last period wrong and she must be 2 weeks younger than I thought, but I knew that wasn't possible as dh wasn't here and then we didn't have sex!

BUT, and I think this is a very under explored part of pregnancy, dc1 went to 42 weeks, ddc was induced at 38 weeks (I was ill) and they were shocked at how much of the white stuff she had and all said she was at least 2 weeks younger than I said (she was the one I knew the dates for)
dc3 went to 42 weeks.

It seems as if my pregnancies are 42 weekers. This it seems is not uncommon, but doesn't seem to fit the 'rules'