Calculated EDD based on scan or ovulation?

[growingweeble]

When I had my dating scan it estimated my due date was 5 days after the dates I'd estimated based on when I ovulated. I was having difficulty conceiving so temping and doing these tests daily. So, I know when I ovulated and conceived.

I wasn't bothered about this until I got to my EDD. Now I'm getting inpatient and not sure when I should start worrying about the baby being late.

Does anyone know which is more reliable? Knowing when I ovulated is not the same as knowing the date of my LMP as that can obviously be subject to inaccuracies as people ovulate at different times in their cycle.

My midwife is being very vague about which date to use.

My cycles were 31 days (have never ever had 28 cycle) yet my EDD is based on LMP - didn't have scan until 13 weeks and they said dates not different enough to change. So I think of it as I have a due week - Monday - based on LMP and Thursday of that week because I probably ovulated later etc - although I have no idea when I ovulated as I wasn't keeping track.

When was the dating scan? - scans at 6-8 weeks are v reliable (within a couple of days) in terms of dating. If the scan was at 12-14 weeks then it can be around a week out.

Its an estimate either way - some people have scans that exactly agree with their dates and then the baby still comes 2 weeks late.

Personally I would think the ovulation date is more accurate - (unless you had v early scan) but lots of babies come a week late (in some other countries they actually consider pregnancy is 41 weeks not 40) so I never think of myself as being late until a week after EDD.

Good point about fertilisation /implantation. Made me realise that my ovulation dates probably are more accurate as they time better with a positive pregnancy test. I don't think ovulation, fertilisation, implantation and then hormone levels could get to a level that a pregnancy test could pick up within 4 days. Could be wrong, but 9 days sounds more reasonable for that.

Also those POAS ovulation tests show ovulation is about to occur (ie you get the hormone surge BEFORE ovulation) - however if you are going by temp rise you get that after ovulation.

No point in thinking about it too much, think of it as a due week!

Easier said than done!! 5 days when you're 40

• weeks is a looooong time, nevermind the fact that I should be getting extra monitoring if I get to 42+ weeks.