Calculation of how far along in the pregnancy

[fornight]

I wonder why is it always calculated from the first day of your last menses?

I just pleasantly discovered I was pregnant today. I thought I should be 2 weeks plus pregnant since my ovulation date was around 2 weeks plus ago. But when I use the calculator on the Web or app, they said I am 4 weeks pregnant? ?? Why??

[ellarose3]

So I havi a question. I need to have the maternal 21 test and it's done between 10-12 weeks. the doctors Go by lmp but I know I got preggo 2 weeks later. They said to schedule the test for 10 weeks and are going with my lmp date. I'm afraid the test won't work  

[KelseyNicole93]

Yep! Based on your LMP!

[Dragonwings]

Congrads! Drs will go by your last period because not everyone know their exact ovulation date. Average ovulation is 2 weeks after your period so the days up to the average 2 weeks is counted as fertilization.

[fornight]

I have forgotten to intro myself. I just joined today. 39 yrs old and first pregnancy.  We have been ttc for 5 months.