Hi all, I am just posting because I'm so confused. My apps and nhs website ask me to put in the date of my last period which was 11.10.15 and say that I'm 7 weeks, however I got married 6 weeks ago on 15.10.15 and can not have gotten pregnant before that. My app on my phone says I ovulated around the 23rd and my clear blue digital test says I am 3+ I am so confused as to how many weeks pregnant I will be??
Until you have a dating scan (12 weeks) date of your last period is how your due date is worked out. Pretty sure clear blue tests only go up to 3+ and if I remember rightly they date from ovulation not lmp so you would need to add 2 weeks to any clear blue reading.
Being 7 weeks pregnant means that you concieved about 5 weeks ago so you will have got pregnant after your wedding, does that make more sense? Pregnancy is dated from the last date of your period as that is the only certain thing before you have a scan, but you would have actually released an egg until two weeks or more after that.
The doctor will tell you how far you are actually along when you have the dating scan.
As for why they use it... a "normal" period they use is 28 days long. In that example you ovulate on day 14 (CD14) and 14 days after that your period starts. Many people don't track their actual ovulation (OPK's or temping for example) so don't know when they ovulate, and some don't ovulate exactly on CD14. Due to that, doctors just use the most obvious day that most people keep track of (the day you bleed - LMP) as the date to use as the "first day of pregnancy".
For myself, as an example, that wouldn't work well for me because I ovulate on CD20. So according to the LMP method, doctors would consider me a whole extra week pregnant compared to someone with a 28 day cycle (until the dating scan). I would need to keep that in mind because my "7 week scan" would actually only be 6 weeks which might not show a heartbeat yet (scary stuff).
The Clearblue dating tests go by HCG levels, which starts being generated as of the day of implantation (which can be around 9 days after ovulation). Those tests approximate the number of weeks of HSG hormone you have (so how many weeks you been pregnant, biologically speaking). This isn't always reliable though because multiples can mean higher HSG, and some people have low HSG levels (my friend does so her HPT's take longer to get a positive).