How can calculate due date with irregular periods?

[Hybrid]

We had great news this evening with a positive test which was quite a shock after 2 negative tests about 10 days ago.

After having the mirena coil removed 6 months ago, I've had incredibly irregular periods and just wondered if there was any way I could roughly work out when my due date would be.

I had what I thought was my period on Monday but actually, it just lasted a day and (with the test result) was probably an implantation bleed. I also had some slight spotting a few days before this.

Would this help with dates or do I just have to wait until I get scanned to have any idea?

Any advice would be most appreciated!

Fristly congrats!
With me, it was cerazette I came off, so periods were allover the friggin place. Anything form 27-54 days. They used my LMP anyway to begin with but said they would review and change dates if needed at 12wks scan as my periods had be so erratic, and according to tests (see below) I should be a few days behind LMP as I ovulated late.
There's a good chance they will use the beginning of your last bleed, unless it was a very long time ago, regardless of any other info.
I also had the luck of having done an ovulation kit for the first time that month, so knew roughly when I'd ovulated. Also had only DTD once around ovulation time (visitors, DH away etc), also narrowing my dates (I tested over 10days) so if you have anything like that that will help the MW then mention it.
It is possible for pregnancy tests to not work until a good 4-5 days after your 'missed' period so, if the bleed you did have was a breakthrough (thus missed) you'll probably be about 4-6wks (Someone please correct my maths if needed)?
As for me, well, I measured 1 day out at the scans...so well, who knows!
I wouldn't worry too much about it for now. But, well, I love reading the foetal development stuff and just wanted to know what LO was doing. For those early days I went with as 'young' as possible for some reason...

Thanks so much for the reply - much appreciated! It sounds like you had a lot more information to go on than I do but like you say, no need to worry about it for now. Just nice to have a bit of an idea when you might be due/how far along you are etc.

It still doesn't seem real at the moment and I keep thinking it's the dodgy Sainsbury's own tester that might be tricking me!

Thanks again

Hehehe!
I kinda know the feeling. BUT by 6 wks I was so exhausted I coudl barely stay awake if I wasn't actually moving somewhere and really work was about all I could manage in terms of facing the world. That and unbelievably sore boobs were getting in the way everywhere...and ....argh cats just vommited on the remote! ...
Anyway, some places combine booking with the 12wks appointment, so you might want to phone the MW soo, explain waht's going on and see waht they say!

I got pregnant while breastfeeding and hadnt started my periods so had no idea how far along i was. I was sent for a scan a week after seeing the gp.