If I have had an irregular menstural cycle my whole life, is it still safe to use the first day of my last period to estimate the age of the baby? I read also that you are fertile for 4 days before your ovulation day? If I am 5 weeks 5 days pregnant based on my LMP what day did I conceive? What are the chances I could have conceived before or after the date the doctor has giving me? This is because I'm not sure who the father of my child is

When determining gestation, the first day of the LMP is what is used. This is because, unless medical assistance was used, there is no way to tell exactly when someone ovulated and conceived. This is also partly why you have a window of several weeks at the end of a pregnancy for a baby to be born in that is considered "term".

Since your cycles are irregular, this can effect the determination of how far along you might be. You will get a better idea when you have an ultrasound done and they can take some measurements of baby.

While you may be fertile for a few days leading up to ovulation, you can only become pregnant when you actually ovulate. The days before are when sperm can survive waiting for the release of an egg, otherwise they will not survive long.

It is generally assumed that you would have ovulated about 2 weeks after the start of your last period. However, many women will ovulate a bit before or a bit after this time and since an egg is only viable for about 24 hours max, even just a day can make all the difference.

If your dates of being with different people are too close to call, the only way you will know for sure who the father is would be to have DNA testing done.