Pregnancy is not timed from conception but from the first day of the LMP (Last Menstrual Period). The reason for this is that the time of conception cannot be accurately determined as ovulation time may vary from one individual to the other.
Hence, two weeks after conception, you are still called 4 weeks pregnant on the basis of your LMP. This is now accepted as a general way of timing pregnancy.
If your LMP is not known, an ultrasound scan is usually performed to determine the EDD.
An average pregnancy lasts for 40 weeks after LMP although it could range from 37 to 42 weeks.
Pregnancy is divided into 3 trimesters, each consisting of 3 months. The first trimester includes weeks 1-13, second trimester weeks 14-26 and the third trimester weeks 27 until birth.