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Congratulations! You're pregnant. Or perhaps you're looking ahead and want to familiarize yourself with what to expect during pregnancy. Read on for a description of how pregnancy progresses and answers to your questions about the terms used to describe the different stages of pregnancy.

How long does a normal pregnancy last?

That depends on which date you use as the starting point. You're not actually pregnant until the day you conceive. But doctors and nurses find it more convenient to start counting from two weeks earlier on the first day of your last menstrual period (LMP). Remember, the LMP is the period that took place just before your first missed period.

On the average, a normal pregnancy lasts 266 days (38 weeks) from the day you conceive and 280 days (40 weeks) from the first day of your LMP.

Wait a minute! That's almost 10 months! That can't be right!

Actually, if you take 38 weeks and divide by four to get 9 1/2 months, or divide 40 weeks by four to get 10 months, you're calculating lunar months, which are shorter than calendar months.

A lunar month lasts only 28 days, or four seven-day weeks. Most calendar months last for 30 or 31 days. That's why it sounds as if there's more time involved when we count in lunar months although it's not actually true. When we talk about a 40-week pregnancy, we're including the two weeks before the baby was actually conceived.

So depending on when you start counting, you can expect your pregnancy to last 38 or 40 weeks, or about nine calendar months.

Why do my doctor and nurses use a 40-week timetable?

As you proceed through your pregnancy, your health care providers will always want to know which week of pregnancy you've reached while everyone else is asking what month you're in. Why do they keep track of pregnancy time in such a confusing way?

Using a 40-week timetable and keeping track of specific weeks are important medical conventions that allow doctors and nurses to describe and track pregnancy consistently. When medical professionals use the same conventions and terminology, it means that everyone knows what everyone else is talking about, even when different patients coming from very different locations are involved.

Calculating pregnancy from the first day of the LMP provides the gestational age of your baby. This is, by far, the most common medical convention doctors and nurses use for expressing how far along your unborn baby is. However, since the date of conception follows the first day of your last LMP by about two weeks, your baby is actually two weeks younger than his or her gestational age.

Week 1, Week 2, Week 3

In the timetable above, Day One is the first day of the mother's LMP, and Day 14 represents the date of conception.

What are trimesters?

The word "trimester" comes from a Latin word meaning "three months long." The three trimesters of pregnancy are a way of dividing up the 40 weeks into smaller, separate stages when the mother and baby are encountering different types of pregnancy events.

The first trimester is usually defined as including weeks one to 12 (counting from the first day of the LMP). During this period, your child grows very rapidly, forming all of the major organs and body structures, and size, though increasing, is still relatively tiny.

The second trimester may be defined as including weeks 13 to 26. The third trimester begins at week 27 and continues to term. As the second and third trimesters progress, your child's organs will mature to the point where your baby can survive as an independent being outside the womb. Your child also increases in size and, particularly toward the end of the third trimester, develops the rounded fat deposits characteristic of a healthy, cuddly baby.

Weeks? Months? LMP? How can I keep all this straight?

Are you tired of counting weeks off calendars? Converting weeks to months and vice versa? If you remember the date of the first day of your last menstrual period and would like a little help keeping all this straight, check out our Due Date Calculator.

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External Source KidsHealth - Nemours Foundation

This article was reviewed and updated June 2007.