40 Week Pregnancy Timetable.

Pregnancy is based on a 40 week timetable. Using this method, you actually become pregnant during the third week. Details of your pregnancy are discussed week by week beginning with Week 3. Your due date is the end of the 40th week.
Each weekly discussion includes the actual age of your growing baby. For example, in the Week 8 chapter you'll see the following:

Week 8 (gestational age)
Age of Fetus 6 Weeks (fertilization age)

In this way, you'll know how old your developing baby is at any point in pregnancy.
It's important to understand a due date is only an estimate, not an exact date. Only 1 out of 20 women delivers on her due date. It's a mistake to count on a particular day (your due date or an earlier date). You may see that day come and go and still not have your baby. Think of your due date as a goal a time to look forward to and to prepare for. It's helpful to know you're making progress. Understanding how time is recorded during pregnancy helps.
No matter how you count the time of your pregnancy, it's going to last as long as it's going to last. But a miracle is happening a living human being is growing and developing inside you! Enjoy this wonderful time in your life.

Your Menstrual Cycle
Menstruation is the normal periodic discharge of blood, mucus and cellular debris from the cavity of the uterus. The usual interval for menstruation is 28 days, but this can vary widely and still be considered normal. The duration and amount of menstrual flow can also vary; the usual duration is 4 to 6 days.

Two important cycles actually occur at the same time the ovarian cycle and the endometrial cycle. The ovarian cycle provides an egg for fertilization. The endometrial cycle provides a suitable site for implantation of the fertilized egg inside your uterus. Because endometrial changes are regulated by hormones made in the ovary, the two cycles are intimately related.

The ovarian cycle produces an egg (ovum) for fertilization. There are about 2 million eggs in a
newborn girl at birth. This decreases to about 400,000 in girls just before puberty. The maximum number of eggs is actually present before birth. When a female fetus is about 5 months old (4 months before birth), she has about 6.8 million eggs.

Some women (about 25%) experience lower abdominal pain or discomfort on or about the day of ovulation, called mittelschmerz. It is believed to be caused by irritation from fluid or blood from the follicle when it ruptures. The presence or absence of this symptom is not considered proof that ovulation did or did not occur.