0 WEEKS PREGNANT: NOT PREGNANT YETYou are not actually pregnant yet even though the pregnancy calendar has started.
The pregnancy calendar begins before you are pregnant.
Pregnancy weeks are counted from the First Day of the Last Menstrual Period (LMP) because the LMP date is more likely to be known than the date of conception.
Pregnancy lasts about 40 weeks (280 days) from LMP. These 40 weeks include approximately two weeks (from LMP to conception) when you are not yet pregnant.
14 days after First day of period = Approximate conception date
266 days after Conception date = Estimated due date
280 days after First day of period = Estimated due date
Even if you know the definite time of intercourse, the actual timing of conception is harder to pinpoint because conception doesn't happen until ovulation has occurred and the egg has been fertilized by a sperm. This could be hours after intercourse or it could be days after.
Using LMP as the start date for pregnancy is a time honored, simple, and reasonably reliable way to estimate the timing of your pregnancy. Using this method, you can have a good idea of how pregnant you are and when your due date is likely to be.
Age of pregnancy based on LMP is known as Gestational Age. This method adds approximately two weeks to the true age of your developing baby since the timer begins before you are actually pregnant. For example, when you are 12 Weeks' pregnant, the fetus is about 10 Weeks' old (Fetal Age).
In general, when someone mentions age of pregnancy, they mean Gestational Age.
Ultrasound: In modern obstetric care, measurements from ultrasound scans may be used to establish a due date and gestational age. If a due date and gestational age have already been calculated based on LMP, they may be confirmed or revised following your ultrasound scan.