Expert: Catherine Beier MS CBE - 1/10/2009

Question
QUESTION: I know!  I thought it was odd too.  I couldn't remember exactly when my period was so on 10-14-08 i had an ultrasound and based on it he told me i was six weeks and three days.  So, how accurate are ultrasound due dates?  Also, i just had my second trimester ultrasound done and he still said June 6th was my due date.  Its a week early if i did in fact get pregnant on September 20th, so I was just kinda confused on how accurate this stuff is.  
When would i have conceived in order to be due June 6th?  This doesn't make much sense to me.

ANSWER: Casey,


Typically, ultrasounds between 8-10 weeks are the most accurate way to date a pregnancy, usually within less than 2 days error.  They are less accurate earlier in pregnancy and much less accurate after the first 12 weeks, being off by as much as 2 weeks in either direction.


However, with all the information you've provided, none of the dates match up.


Based on the first ultrasound, you most likely conceived around September 13th.  


This doesn't correspond to either of the dates of intercourse you provided.


So, something's amiss here.  Is it possible that you misremembered the dates of intercourse?


If not, then a paternity test is your best answer.  I'd still wager that your husband is the father, since you indicated you had a full period after your encounter with the first man and the dates you've provided are closer to him being the father than the other man.


But since nothing matches well, I can't give you the peace of mind I'd hoped to.


Sorry!
Catherine

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

QUESTION: Okay, i went to the dr. for a yeast infection yesterday without my husband and was able to express my concern.  His response was that the difference between 6 weeks and 8 weeks is tremendous.  He said if i conceived on August 30th, at my first ultrasound on 10-14-08 i would have been 8 weeks and had a much bigger baby; he couldn't even stretch the due date any to make it fit with the August 30th encounter.  He said that September 20th is possible because i have a little longer cycles and fetal heartbeats can be visible at 5 weeks. (which is what i would have been on the day of my first ultrasound if i conceived on the 20th.)  I guess i'm just looking for another opinion.  Do you agree that the difference between a 6wk and 8wk baby would be as noticable as he said?  As far as the dates, i could be wrong about the one with my husband.  It could have been the weekend before September 20th.  However, I'm 100% sure the other guy was on the 30th of August.  So, you do think its VERY unlikely to be the other guys?  Also, if the baby decides to come early or late, does it mean my due date was off or does it just mean he comes when hes ready?

Answer
Casey,

Yes, I agree with your doctor that the difference between 6 and 8 weeks is tremendous.  There is little mistaking the two.


He's also correct about seeing fetal heart beats at 5 weeks.

I think it very unlikely to be the second man but again, only a paternity test will be the definitive answer.


And yes, your baby will come when he's ready.  This doesn't mean the "due date" was off as it's just an estimate anyway.  Less than 5% of all babies are born on their due dates.



Best,
Catherine