I am currently in a horrible, very difficult predicament. My partner is currently pregnant but she was sleeping with me and a another male at the same time. Initially I took the view that it was a 50/50 chance as to who the father is. After researching and learning more about conception, sperm survival, calculation of conception from EDD etc I have realised this baby is more likely to be mine and in fact it seems to be almost 0% being this other guys. My partners estimated due date is 11th January 2012. That indicates her LMP was 6th April, and based on an average female cycle (has to be average because my partner was extremely irregular and can't recall having her last period) her ovulation window would of been the 6th plus 12-14 days so that's the 18th-20th April. I was sleeping with her from the 14th to the 19th April. She slept with this other guy on the 9th April. Based on these calculations it seems highly unlikely to me. The only way would be if the foetal size/due date was measured inaccurately but as she had her 1st scan very early I believe the crown rump length measurement is accurate to within 3-4 days if the scan is between 6-12 weeks. I know the accuracy diminishes the later the scan. But based on the above what do you think? I would really appreciate your opinion on the matter it is such a horrible, youngsters typical cliche situation to be in and your views would be greatly welcomed.