Pregnancy Timeline & Ultrasound Dating: Understanding Gestational Age

Question
My last lmp was on June 8th 2012. I had unprotected sex on June 12th and June 14th with partner number 1. On June 17th, June 24th and July 1st I had unprotected sex with partner number 2. I found out on July 16th I was pregnant. I went to my first transvaginal ultrasound on August 18th which dated me at 10 weeks. Is the ultrasound going by gestational age or by fetus age? I know there is a difference between and 8 week and a 10 week fetus, but why would they say I have 10 weeks, when I actually think I have 8 weeks? Please advise? Partner #2 who I actually believe is the father does not believe he is if they say I am 10 weeks pregnant, because 10 weeks ago he did not have intercourse with me. On the pregnancy calculators I have entered both dates my lmp of June 8th 2012 and it gives me a due date of 3-15-13 which is the due date the doctor gave me. I also entered on the pregnancy calculator conception date which I think was the 24th of June and it gives me a due date of 3-17-13. Both of these dates put me at being 10 weeks now, even though there is a two week difference. Please advice. Thanks!

Answer
Maria,

Pregnancy length is calculated as 40 weeks from the first day of your last period.  Since conception usually occurs about two weeks after your last period begins, you are not technically pregnant for the first two weeks of your pregnancy term.  So, when your doctors said you are 10 weeks, they mean you are 10 weeks from the first day of your last period and the fetal age is 8 weeks.

So, if your LMP was 6-8-12, you most likely conceived around 6-22-12 and are due 3-15-13.  You could have gotten pregnant from intercourse in the five days before 6-22, or anywhere between 6-18-12 to 6-22-12.


Best,
Catherine