And sperm can hang around to fertilize for three days after intercourse. But that's the way your practitioner (and everyone else) will most likely count the weeks.

So even if you know the day and time your egg was released, you don't know for sure the date sperm and egg actually got it together.) Which explains why the first day of your pregnancy (at least when it comes to dating it) begins on the first day of your last menstrual period (LMP) — even though your baby is technically about two weeks younger than that (most women ovulate and conceive two weeks after their LMP) and will only have clocked in 38 gestational weeks by the time your pregnancy has reached the 40-week mark. Keep in mind that women with irregular cycles may have difficulty calculating their due dates (the LMP method is far from a perfect science), so if you fall in that camp talk to your doctor to get the best estimate of your due date.

A common result is to be pressured into a pitocin induction of labor, which when done on an unripe cervix has a very high failure rate, can be quite painful, and often ends in a C-section for "failure to progress." In addition, babies who are induced due to incorrect dating may be born earlier than nature intended, and can have immature lungs and other problems, needing special care.

It is in Mother's and Baby's best interest to date the pregnancy correctly; this procedure is of critical importance!

Health-care providers define the stage or length of pregnancy differently than many people might think. Logically, one might imagine we represent a pregnancy by how much time has elapsed since conception.

Instead, we most commonly date pregnancies from the first day of the last menstrual period (LMP).

As a result, most people, including health professionals, will date your pregnancy based on your LMP (last menstrual period) instead.

To work out how many weeks pregnant you are by this measure, try to remember when your last period started.

This will give you an estimated date of conception, which will usually be about two weeks after the first day of your LMP.

One reason for not dating pregnancies from the day of conception is that we cannot know that day exactly (excluding cases of assisted reproduction), but we can know the first day of LMP, based on what a patient reports to us.

Also, when providers all use the same LMP date, we are using one system that provides a standard convention.

As part of your antenatal care, your midwife will offer a dating scan that will give you a more accurate date for the birth of your baby.

This calculator gives a rough idea when your baby may arrive.