QuestionI had an ultrasound done May 1st. The sonogram tech (my mother) told me I was 7 weeks on that day. I was planning on not keeping the baby so it was very important that first time to get the measurements correct. Since then I have had one at 9 weeks which showed 9 wks 1 day. 11 weeks which showed 11 wks 3 days and one at 13 wks which showed 13 wks 3 days. They all seem to say pretty much the same thing. I would like to know how accurate the first one was at telling me how far along i was. The estimated conception date from the 1st sono was March 12th. I do not remember my last period as they are so irregular I do not keep track. I slept with my ex and another guy that same month and am trying to determine who the father is without getting any kind of paternity test. If the first ultrasound is pretty accurate I know who it is...if not, it could be the other guy. How many days off could the first one have been if the other 3 have all been pretty accurate with the first?
AnswerDear Hollie,
Ultrasounds done within the first trimester are considered to be very accurate in dating pregnancies. In general, ultrasonography dating of a pregnancy is more accurate than dating using the date of the mother's last menstrual period. The biometric crown-rump measurement (CRL) of the six to 12-week fetus provides the most accurate measurement - within three days of true gestational age. From 12 to 18 weeks gestation, other anatomic structures are typically measured and biometric measurements are usually accurate to within a week of gestational age. After 26 weeks gestation, fetal variability has greatly increased and the dating provided by biometric measurement is less accurate, but the method is still used. By late in the third trimester, measurements are accurate only within a two-to three-week window.
According to my calculations, if you were measured to be at 7 weeks gestation on 5/1/09, your estimated date of conception would be on/about 3/27/09. Your estimated due date would be 12/17/09. I'm not sure where your March 12th potential conception date came from with that first ultrasound. There may have been some confusion related to pregnancy dating, which is often the source of the discrepancy... The calculation of gestational age, or pregnancy dating, is not actually based on the conception date. It is based on the LMP date. Always. Even if the LMP date is not known and even if the dating is determined via ultrasound. The ultrasound will automatically calculate the gestational age determined by the measurements of the fetus or gestational sac, etc,..., but that is all based on norms for gestational age calculated from LMP. The actual "full-term" confinement of pregnancy is 40 weeks from LMP to delivery. It is assumed that conception takes place two weeks after the LMP date because most menstrual cycles last 28 days, with ovulation occurring on/about CD 14-15. I know... this is very confusing for women, but that is just how it is. Therefore, your 3/12 date would have been your likely LMP date, NOT your potential conception date.
I hope this has helped you and answered your question. I wish you well.
Brenda
