Expert: Catherine Beier MS CBE - 9/30/2011

Question
Hi catherine, im a little confused to conception timing and ultrasound dates. I found out i was pregnant in the month of june, that is when i went in for an ultrasound finding out i was 6w6d at the time. Im not 100% sure as to when my Lmp was but when i think back it was most likely April 20th even though G.A for ultrasound puts my Lmp to fall on April 28th with a due date of 2/2/2012. Can my ultrasound be off by a week that early on? And if so when would my conception date be because i was with a guy i was dating on April 26th and then my Exboyfriend May 7th so who would be the likely father? The last ultrasound i went in measured G.A to be 18w2days on Aug 31st putting me 3 days bigger. Thanks for your help!

Answer
Hello Sal,

No, an early ultrasound cannot be a week off - a 6w,6day old fetus looks very, very different from a 7wk,6day one.  The difference in dates would mean that you ovulated later in your cycle, not that the first day had to be the 28th.  Calculating due dates is done off a 28 day cycle with ovulation on day 14, so for women who do not have this (which is most women) the date they give for your LMP probably isn't accurate.  If it differs, this means you either ovulated and conceived early or later than day 14 in your cycle.

So, based on your information, you most likely conceived around May 12th, meaning that your ex would be the father.  The other incident was to far away from conception to possibly make him the father.