Expert: Catherine Beier MS CBE - 9/20/2008

Question
QUESTION: hi my name is katie. i was wondering if you could help me. i had an ultasound that said im 7 weeks 2 days pregnant about a week and a half ago. if i went back 7 weeks 2 days it was during my last normal period(7/18).my cycles are 28-30 days. it was 2 days late but 7 days long. i had sex 8 days before my period. is there any way i could of got pregnant then?? i did have sex when i was supposedly ovulating in the beginning of september. i have also been told that an ultrasound machine adds two weeks. its all very confusing please help

ANSWER: Katie,

To find the date you most likely conceived, you would count back approximately 5 weeks, 2 days if your ultrasound said you were 7 weeks, 2 days. So, your probable conception date was likely around August 1st.


Due dates are typically calculated as 40 weeks from the date of your last period, which means that the first two weeks or so, you aren't pregnant yet.  This is confusing and accounts for the extra two weeks.



Best,
Catherine

---------- FOLLOW-UP ----------

QUESTION: thanks so much!!!! does every ultrasound add two weeks?? its so weird to me.. ?!

Answer
Katie,

The short answer is yes.  One of the reasons is that the average menstrual cycle is 28 days with ovulation occurring about 2 weeks before your next period.  It's for this reason that due dates are often calculated from the date of the last period to make the math a bit simpler.  The same principle is applied using ultrasound in that two weeks are added, but they are more accurate because they measure the size of the baby rather than just looking at dates.  The downfall of the LMP due date is that it won't be accurate if you have long and/or irregular cycles or ovulate early or late in your cycle.



Best,
Catherine