Expert: Dorothy H, LCCE - 5/20/2009

Question
QUESTION: I need to know the margin of error on ultrasounds. I want to know if it is medically possible i could have conceived March 14 2009. Here is my question.
I had unprotected sex March 14th. On March 29 at night i had a period that lasted until March 31. About three days but lighter than usual. It scared me so i took a pregnancy test April 1st that came out negative. On April the 11th through the 14th i also had unprotected sex. On April 28th i received a positive pregnancy test.
On may 1st i had an ultrasound done that said i was 4 weeks along. I scheduled another ultrasound for May 15th approximately 2 weeks later that said i was 6 Weeks and 3 days along.
My adhesion hinges on these dates. I need to know if this baby was conceived March 14th or in April. How off can an ultrasound really be?

ANSWER: First trimester ultrasounds are accurate in determining gestational age within 5 days.

Remember, when they are calculating gestational age, they date the pregnancy from two weeks before you conceive.  (generally the first day of your last period) The actual conception date is approximately 2 weeks after that. It is confusing, because when you are 4 weeks pregnant, you really conceived 2 weeks before, not 4 weeks before!

So an ultrasound on May 1st showing you were 4 weeks pregnant and an ultrasound on May 15 showing you were 6 weeks 3 days pregnant fits right in with your last period being on March 29 and with your baby being conceived mid-April.

There is no chance that you conceived March 14.



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

QUESTION: Great thank you. I was also concerned that she really didnt hear a heartbeat- is this normal? and second would a tilted uterus affect any dating with  an ultrasound (transvaginal).

Answer
A tilted uterus would not affect dating of the pregnancy.

With an ultrasound before 7 weeks, finding the heartbeat is hit or miss...sometimes they can pick it up, sometimes they can't. So yes, it is normal with early ultrasounds to not be able to pick up the heartbeat.