Confused about due date

I was wondering what due date should I go by my ultrasound due date or by the first day of my last period? Because they are giving me 2 different due dates. The ultrasound said when I was 17 weeks pregnant that the baby was measuring 20 weeks so my new dute date was Jan 25. But my first day of my last period is giving me a due date of Feb 9. Can it be 3 weeks off like that?? PLEASE HELP!! Its my first baby and I am so confused!!! The doctor is mo help. Ive only had that one ultrasound. They wont give me another one. But the Dr. did say that they baby was a big baby when I went in last tme.

[Keith Downing, MD]

Hello,

Good question.  It is not uncommon for pregnancy dating by ultrasound (U/S) to differ from dating determined by the last menstrual period (LMP).  The earlier the u/s the more accurate it is in determining the age of the pregnancy.  U/S is often used to confirm a woman's dating and if there is a discrepancy, the u/s dating is used.  General guidelines are that if a 1st trimester u/s differs 7 days or more, or a 2nd trimester u/s differs 14 days or more from the due date based on the LMP, the u/s dating is used.  If the due date is within the 7 or 14 day window the LMP is used

Also, if a woman is not certain about her LMP it is usually best to use u/s dating.

In your case, your LMP gave you a due date of 2/9/08, while your u/s dating at 20 weeks gave you a due date of 1/25/08.  Based on the above rule you fall just outside that two week window, so I'd go by your u/s dating.  I hope you can appreciate that pregnancy dating is not an exact science.

Best regards,

Dr. Downing

[mamaofonetrying42]

the u/s due date is the most accr. date to go by. And thats the due date your doc will go by as well.

[devildogwife86]

Its crazy though... My dr gave me an answer telling me that the baby will come when the baby is ready anywhere between Jan 25 and feb9. That was his answer!

[mamaofonetrying42]

Thats a little crazy he gave to a 2week time period.I would go by the u/s due date but my guess of way your doc did that is because some doc let you go 2week over due,

[stablethrower]

Your story was similar to my delivery last year.  My doctor had 3 due dates for me.  One being the beginning of Oct, One being October 30, and the last when I went on my due date of October being November 16 according to the ultrasound.  My baby measured small throughout the whole pregnancy.  She ended up weighing 6 pounds 81/2 ounces.  I kept telling the doctors that I was losing a lot of fluid for over a month.  Finally when I went in on my due date and my regular doctor was back and he checked me I had to be admitted to the hospital the next day to be induced.  Other wise I would have had a dry birth.  I was induced for 2 days.  I ended up having to have a c-section on November 1 that night.  I think the ultra sounds are sometimes off.  Just like now I am pregnant and I am schedule for a c-section on Feb 1, but I already told the doctor and them that I think it is going to be before then.  I have not had a period since March and they are saying that I got pregnant in May according to the ultrasounds.  We will see though.  I went on Wednesday for another ultrasound and it now shows that I am measuring about 31 weeks but they are saying that it is still 29 weeks.