Gestational Age Determination
Gestational age determination involves procedures and tests used
to check the age of a fetus during pregnancy.
Who is a candidate for the procedure?
Once a woman learns that she is pregnant, it is important to find
out how far along she is in the
pregnancy. Knowing the age of the fetus is key to good health care
during pregnancy. Traditionally, the due date was based on the following:
date of the last menstrual period
an early pelvic
exam
measurement of the size of the uterus
quickening, or feeling the baby's first movements
All of this is still part of routine obstetrical care.
However, these methods are very unreliable for some groups of women, such as:
overweight or obese
women
those who are unsure of their last menstrual period
women who have been taking oral
contraceptives
women who have irregular
menstrual
cycles
How is the procedure performed?
There are several methods for determining gestational age. A
pregnancy ultrasound uses sound waves to examine and measure the
fetus. Undergoing an ultrasound procedure early during the first trimester allows a more
accurate due date. Later, ultrasounds tell more about the growth of the fetus
and the placenta.
Some ultrasound measurements are as follows:
Crown-rump length. This is the distance from the top of the
fetus' head to its buttocks. Crown-rump length is currently the most accurate
ultrasound pregnancy-dating method. When it is used in the first trimester,
babies are born within 5 to 6 days of the predicted due date more than 90% of
the time.
Biparietal diameter, or BPD. BPD measures the distance
between two skull bones. This measurement may be taken anytime after the 12th
week of pregnancy.
Abdominal circumference. This is a measurement of
front-to-back and side-to-side lengths of the baby's abdomen. It is calculated
by a computer and may be used after the 14th week of pregnancy along with the
distance between the skull bones.
Head circumference. Measuring the distance around the
baby's head is another way to track growth.
Fetal limb measurement. Checking the length of the baby's
femur, which is the long leg bone, correctly predicts the due date within 7
days.
Transcerebellar diameter. This is the measurement of the
distance around one part of a baby's brain.
Mean ultrasound age. This measurement averages all the
above measurements to arrive at a reasonably accurate due date.
The following ultrasound measurements are used less often:
chest diameter
foot length
inner and outer orbital, or eye, distance
length of the collar bone
length of the lower jaw bone
liver size
thigh diameter
The size of the woman's uterus is also used to determine
gestational age. During the first trimester, the length of a pregnancy can be
determined by a pelvic exam. At 8 weeks, the uterus is felt just at the pubic
bone. At 12 weeks, it grows over the pubic bone into the abdomen. At 20 weeks,
the top of the uterus lies at the level of the belly button. After that point,
it is measured in centimeters. Each centimeter corresponds to the week of
pregnancy. Twenty-six centimeters, for example, equals 26 weeks gestation. If
there is more than one fetus, this calculation does not apply.
Other methods for determining the age of the fetus include the
following:
fetal heart tones, which can be heard on a small handheld ultrasound unit
pressed against the mother's abdomen
a pregnancy calendar or calculator, which calculates due date based on the
last menstrual period
quickening, or movement of the fetus, that is usually felt between 16 and
20 weeks
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