preggo test science for TTC geeks

[PassTheAfterEights]

Full disclaimer - I love this stuff but know it does some people's heads in so no offence if it's not for you (or everybody else already knows this and it's just me)

Stumbled onto a 2008 research paper on hCG after implantation & thought it was fascinating and helped me understand a bit more and hope it will now save me a tonne of dough on early testing

Started a new thread so it doesn't clog up any existing buses or discussions, hope that's OK.

www.ncbi.nlm.nih.gov/pmc/articles/PMC5330618/

Key bits -

You can have an existing level of 1 - 5 hCG miu/ml without being pregnant

hCG development varies a lot from woman to woman but as a general rule levels double every 48 hours post-implantation

Implantation can be as early as 6dpo and as late as 12dpo; mean & median is 9dpo

Implantation at 10dpo or later shows a slower rate of increase in hCG levels overall

So, the following examples assume the same 28 day cycle and the same ovulation date but show very different length of time to BFP:

if a woman implants at 8dpo, with an existing non-pregnant hCG level of 1 miu, it would rise as follows

8dpo - 1miu
10dpo - 2miu
12dpo - 4miu
14dpo (AF is typically due) - 8miu
16dpo - 16miu

So a FRER (needs 6.3miu for a line) couldn't show this lady she was pregnant until 13dpo and a one-step (needs 10miu for a line) 15dpo. If she implanted at 6dpo a FRER could read pregnant 10dpo; if she implanted at 12dpo she couldn't show a positive on a FRER until 18dpo, several days after AF was due in a standard cycle.

if a woman who implants at 8dpo and has an existing non-pregnant hCG level of 5 miu, it looks like this:

8dpo - 5miu
10dpo - 10miu
12dpo - 20miu
14dpo (AF typically due) - 40miu

This lady would read pregnant on a FRER at 9dpo and on a one-step at 10dpo. By the time AF is due, most tests would tell her she was pregnant. If implantation occurred at 6dpo, a FRER could show pregnancy at 7dpo; if she implanted at 12dpo a FRER could show pregnancy at 13dpo, or when AF was due.

If you've managed to read this far there's a calculator here that allows you to play with starting hCG levels & earliest possible testing whenmybaby.com/implantation-doubling-calculator.php

Love this sciencey stuff!! Thanks for sharing, now to google how to work out my baseline hcg level...

cakelaur have you seen this?

I love this PassTheAfterEights thanks for sharing!

Very interesting! Thanks for this.

ah bless you ladies thank you, I know it winds some people up overworking this stuff but feel it helps me get a handle on the emotions x

@lavendersblue42 please let us know if you have any luck; not sure if baseline hCG is a consistent thing for each of us month on month or varies a little or varies a lot!

Very interesting, thanks for sharing!

Good to know, thanks!

I never thought of it like that. Now it makes sense as to why I got a BFP at 9dpo but some women don’t get them till later. I couldn’t understand why before x

Me neither @MagicalCreatures, also had a BFP at 9dpo and have been utterly devastated at 11dpo+ in subsequent cycles, going to really try to relax a bit now.

Really interesting, thanks for sharing!

That's really interesting. And explains how I've got early BFPs with both of my pregnancies.

I knew it was related to implantation date, but hadn't considered existing hCG levels in the mix!

(I researched most of the science behind pregnancy while TTC, but didn't stumble on this one!)

PassTheAfterEights thank you for posting, this is really interesting! Made me realise that some tests that are 15- 25 mlu sensitivity really aren’t likely to show anything until you’re late for AF

Ahh no worries x now I just need to remember it while scowling at 10dpo BFN's!

I love that science stuff! Thanks for sharing.

I researched all of this last month, it really helped calm me down so thanks for putting it all together! :)

I think when I looked I saw that you can’t really calculate a baseline HCG, you would need a blood test. Even then I think it varies month to month xx

Thanks this quite helpful because after stopping the depo my cycle lengths have been all over the place so I didn't know when to test 25th looks good for me. I don't know why I didn't look at this before with my almost biological degree (less than 2 months left) 😂

Really interesting, thanks for posting!

@PassTheAfterEights AMAZING stuff!! Love a good behind the scenes... sciencey thing!!
@peachesarenom LOVE IT!!! Defo explains mine this month! Slightly late implantation plus maybe a lower base level of hcg = bfp on 15dpo.

No worries chaps!

@stephisaur v interesting, thank you - I'd wondered after three cycles of early vfl's that don't develop whether sensitive FRER's could measure fractionally elevated base HCG, but it can't be that consistently elevated.

I know this is a zombie, but I've just read it and it's very helpful!!!!+

Haha, @Waakyeandredred just logged onto MN for the first time in about 2y to see your post at notifications so thanks! And about to re-educate myself as we head onto the rollercoaster of ttc again....