Expert: Catherine Beier MS CBE - 11/28/2011

Question
I'd like to know your opinion. I'm not 100% sure when my last menstrual period was so I'm going off ultra sound dates confirmed by my doctor. On 11/18 I was told my baby is 23 weeks & 6 days along in his development. On 11/30 is when I find out my final due date. So, I had sex with two different partners one on 5/29 & the other on 6/4. I understand that it takes time for fertilization and implantation to accrue, but exactly how much time? I also understand that sperm can live up to 3-5 days. So in my case who do you think is more likely to be the father & when did I most likely get pregnant? Btw both men know of & we all agreed on a dna test. I'm just curious to know who is more likely to be the father.

Answer
Aubrey,

Due dates are most accurately calculated between 8-10 week gestation.  Due dates given after the first trimester become increasingly inaccurate as your pregnancy progresses, eventually having a margin of error of up to 2 weeks in either direction.  So, your ultrasound on 11/18 and the forthcoming one on 11/30 could both be off by a week or more in either direction. It is not standard practice to give a "final due date" this far into a pregnancy for these reasons.

Due dates should typically be calculated within the first 8-10 weeks of pregnancy and should not subsequently be changed by later ultrasound measurements unless the later measurements differ by more than 2 weeks from an early ultrasound in the first trimester.

So, the information in the remainder of my answer is based on your being 23 weeks, 6 days as of 11/18, which may not be accurate in itself.  Based on that information, your due date would likely be around 3-9-12 with conception occurring around 6-4-11.  

Given this information, it's equally likely that either man is the father because 1. the dates of intercourse are less than 5 days apart, meaning that sperm from the first man could have still been alive and fertilized your egg 2. pregnancy usually results from intercourse in the two days prior to ovulation and rarely after ovulation as the egg only lives for a matter of hours unless it is fertilized and 3. it takes time for sperm to travel to the egg so even though intercourse with man #2 was closer to your suspected ovulation date, his sperm may not have had time to reach your egg.

So, you'll need a paternity test to know for certain which man is the father. It's 50/50 at this point with the information you've provided.