Expert: Catherine Beier MS CBE - 1/12/2012

Question
I recently found out i was pregnant and did not have a lmp to go by so i got an internal ultrasound that dates me 5 wks 5 days.  I may not have had my lmp date but I was charting my cervix and do no when around I ovulated which doesn't match up to the ultrasound date by 2 weeks. Would you say an ultrasound that early on is more accurate then knowing my exact ovulation date?

Answer
Anna,

The problem seems to be that you aren't understanding how pregnancy dates are calculated. Pregnancy length is calculated as 40 weeks from the first day of your last period (or about 38 weeks after conception, since conception usually occurs about 2 weeks after your period begins if you have a regular 28 day cycle). This means that for the first two weeks of every pregnancy, you are not, in fact, pregnant yet. Confusing, yes, but that is how it is calculated.

So, the ultrasound dates of 5 weeks, 5 days would correspond to 5 weeks, 5 days from the first day of your last period or 3 weeks, 5 days since conception.

In either case, early ultrasound is the most accurate way to date a pregnancy, with an error of measure of +/- 3 days.  The date of 5 weeks, 5 days was from your suspected LMP, not your conception date.

There is simply no way that the ultrasound could be off by 2 weeks.  You didn't specify which direction you thought it was off (making you two weeks further along than you thought, or two weeks less than you thought) but in either case, if you were 3 weeks, 5 days, then nothing would show on the ultrasound and if you were 7 weeks, 5 days, then you would see the fetal pole with arm and legs buds, a beating heart, and a much greater degree of development, etc.  If you were 5 weeks, 5 days, then you would see the gestational sac and yolk sac and maybe also the fetal pole. The heart would not likely be beating yet.

Here are some ultrasound pictures that show what you would see at 5 weeks, 5 days vs. 7 weeks, 5 days and as you can clearly see, there is no mistaking the differences in development between them.  There are no example photos of 3 weeks, 5 days as ultrasounds are not done at that stage for dating purposes as too little development can be seen.


5 weeks, 5 days:  http://www.baby2see.com/development/ultrasound_sonogram/im/5w_yolksac.jpg
7 weeks, 5 days:  http://pregnancyandbaby.sheknows.com/files/ultrasound/7w5d_tanya.jpg



Best,
Catherine