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McKesson Clinical Reference Systems: Women's Health Advisor 2002.2
Estimating Gestational Age
What is gestational age?
Gestational age is the age of an unborn baby. It is
measured in weeks and days, not in months. Because you may
not know exactly when your baby was conceived, your baby's
age is based on the date of your last menstrual period.
Why is it important to know the gestational age?
The progress of a pregnancy can be judged as normal or
abnormal only when the age or your baby is accurately known.
It is very important to know your baby's age if problems
occur and early delivery may be needed. It is also
important to know when a baby is overdue so the health of
the baby can be more carefully watched.
How is gestational age calculated?
The age of your baby and your due date may be calculated
from the date of your last menstrual period. If you have
regular periods and you are sure of the first day of your
last period, your due date is estimated to be 40 weeks from
that day. An early exam of the uterus and an early positive
pregnancy test also help determine your baby's age.
Ultrasound can be used to confirm your baby's age. The baby
can be measured with ultrasound as early as 5 or 6 weeks
after your last menstrual period. This method is most
accurate in early pregnancy. The best time to date a
pregnancy with ultrasound is between the 8th and 18th weeks
of pregnancy.
If you have had a special procedure to become pregnant, such
as artificial insemination or in vitro fertilization, you
will know when your baby was conceived. In these cases
there is no doubt about a baby's gestational age.
It can be hard to determine accurately a baby's gestational
age and your due date if:
- You have irregular periods.
- You cannot remember the date of your last period.
- The baby is unusually large or small.
- The due date based on the last menstrual period does not
correspond with the size of the uterus early in pregnancy
or with the height of the fundus (top of your uterus)
later in the pregnancy.
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