How to calculate gestational age

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Belly size can be an indicator of gestational age
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Knowing how to calculate gestational age will help you know your due date

You are pretty sure you are pregnant. At least, the six pregnancy tests that you took said so.

So when is your baby due? With your obstetrician's guidance, you will learn how to calculate gestational age.

When you go for your first medical appointment, you will be asked the date of your last menstrual period. (Note to women: You are supposed to keep track of this sort of thing.)

Let us say your last period was May 1. The doctor will count back nine months to February 1 and add a week. Your due date is February 8.

Or you can add 280 days to the first day of your last period. If your last period started on (let us make it easy) January 1, add 280 days and you will discover that you are due on October 8.

Another way is to add seven days to the last menstrual period, subtract three months and add one year. If your last period was April 10, 2011, add seven days, which makes it April 17 and now subtract three months. You are due on January 17, 2012.

Of course, the doctor actually has a wheel-like apparatus that makes it easier for her to do the gestational age calculations, as if counting back three months and aging a week is difficult even for the most mathematically challenged.

The reason the first day of the last menstrual period is used to calculate the ETA (expected time of arrival) is because a lot of women do not know when the date of conception was, although some do and can tell you down to the exact second.

Five or six weeks after your last menstrual period (which means you could be in actuality three or four weeks pregnant or a little more or a little less) you will undergo an ultrasound. The size of your baby is measured and this gives a fairly accurate account of the precise age of your fetus, although the gestational age of the baby becomes more evident later on. The ideal time to calculate gestational age is between the eight and 18th weeks of pregnancy.

Women generally, but not always, conceive between the 11th and 21st day after their pregnancy. At least this holds true for women who have a regular cycle. If you have undergone artificial insemination or in vitro fertilization you will know precisely what day you conceived.

A pregnancy normally lasts 40 weeks from the first day of the last menstrual period. However, only five percent of babies are actually born on their due date.

Your doctor will measure your abdomen each time you have an appointment to help her calculate the gestational age of your baby. She measures the distance from the pubic bone up over the top of the uterus to right beneath your breasts. The distance, which is measured in centimeters, is supposed to be equal to the number of weeks you have been pregnant, which is called MacDonald's Rule. This only works from mid-pregnancy until the end of pregnancy, not in the early stages of pregnancy.

If you do not have a measuring tape, use your hand. At 16 weeks pregnancy or four months the top of the uterus is located one-half way between the pubic bone and the umbilicus (belly button.). At 20-22 weeks gestation, the top of the uterus is right at the umbilicus  When you are full term, the top of the uterus is at the level of the ribs.

When you are in between 14 and 28 weeks pregnant, which is the second trimester, an ultrasound is very precise at predicting your due date, within a 10 to 14 day period, based on the diameter of the head  of your baby. Ironically, the farther along you get, the harder it is to accurately predict the due date via ultrasound. The date of arrival can vary as much as three weeks whereas it was pinpointed more closely when you had a ultrasound between 14 and 28 weeks.

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