Gestational Age Determination
Gestational age determination involves procedures and tests used to check the age of an unborn child 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 unborn child 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 maternity 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 unborn child. Undergoing an ultrasound procedure early during the first trimester allows a more accurate due date. Later, ultrasounds tell more about the growth of the unborn child and the placenta.
Some ultrasound measurements are as follows:
- Crown-rump length. This is the distance from the top of the baby's 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 baby, this calculation does not apply.
Other methods for determining the age of the baby include the following:
- the heart tone of the unborn child, 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 unborn child, that is usually felt between 16 and 20 weeks