Expert: Catherine Beier MS CBE - 8/30/2011

Question
I was with guy #1 for 2 years until 4/23 and guy #2 from 4/24 till recently. I am not sure of my last period but I know it was late march, early April. I've never had regular periods and didn't keep track of how far apart they all were. However, ive never completely skipped a period. I went on birth control pills on may 9th and had a positive pregnancy test on June 2nd. On My ultrasound on june 8th I was told I was 7 weeks and 2 days and my due date was 01/23/2012. Then on 8/11 I was told at my second ultrasound I was 17 weeks and 4 days and she told me my EDD is now 01/17/2012. When I asked my nurse practitioner about this concerned about who the father is she told me my lmp was on April, 20th which I'm positive I didn't have one then and that my first due date was right. I talked to her about getting amd aminocentesis for paternity sake and she said no places around my area will do that and its very expensive and could harm the baby. I'm wondering how accurate are these dates and if you have advice on my conception date.

Answer
Hello Kelsey,


Ultrasound during the first trimester is more accurate than later in pregnancy. This is because in the beginning of pregnancy, babies develop at very similar rates.  It is not until after 12 weeks when the placenta takes over supporting the pregnancy that individual factors, such as genetics, diet, health, lifestyle, nutrition, etc come into play and cause some babies to grow larger or more rapidly than others.  Thus, unless a second trimester ultrasound differs in due date by more than 2 weeks, the initial due date should not be changed.

So, the results of your first ultrasound are the ones that should be used to calculate your dates.  Based on this, you most likely conceived around May 4th, making man #2 the only possible father.

If in fact your second ultrasound was correct, then you most likely conceived around April 27th, meaning that depending upon the last date you had intercourse with man #1, if it was less than 6 days before the 27th, then either one could be the father, as sperm can live in the reproductive tract for up to 5 days.  


So, the only way to know for certain is with a paternity test, but my opinion would be man #2.




Best,
Catherine Beier MS CBE