Definition Gestational age determination involves procedures and tests used to check the age of a foetus during pregnancy.
What is the information for this topic? Once a woman learns that she is pregnant, it is important to find out how far along she is in the pregnancy and her "due date." Knowing the age of the foetus is key to good health care during pregnancy. Traditionally, the due date was based on:
the date of the last menstrual period.
an early pelvic examination.
quickening, or feeling the baby's first movements.
measuring the size of the uterus.
All of this is still part of routine obstetrical care. But for some women, such as those who are overweight, those who have irregular menstrual cycles, those who are unsure of their last menstrual period, or those who have been taking birth control pills, these methods are very unreliable. Therefore, more accurate ways to check foetal age and growth are used, such as a pregnancy ultrasound. During an ultrasound, certain measurements are taken. Ultrasound may be used during all three trimesters. Undergoing an ultrasound procedure early during the first trimester allows a more accurate due date. Later, ultrasounds tell more about the growth of the foetus and the placenta.
All of the following methods are used to find or estimate the age of the foetus:
1. Pregnancy calendar or calculator: Normally women are pregnant 280 days or 40 weeks from their last normal period. Doctors use a wheel that calculates due date based on the last menstrual period.
2. Size of the uterus: During the first trimester, the length of a pregnancy can be determined by a pelvic examination. At 8 weeks, the uterus is felt below 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 centimetres. Each centimetre corresponds to the week of pregnancy. Twenty-six centimetres, for example, equals 26 weeks gestation. If there is more than one foetus this calculation does not apply.
3. Quickening: Usually first-time mothers first feel the foetus move between 18 to 20 weeks. For women who have already had a child, quickening generally occurs at 16 weeks.
4. Foetal heart tones: A small hand-held ultrasound unit usually picks up foetal heart tones by 12 weeks.
Ultrasound measurements include:
crown-rump length, is the distance from the top of the foetus' head to its buttocks. This 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 due date predicted more than 90% of the time.
biparietal diametre (BPD), which measures the distance between two skull bones. This measurement may be taken anytime after the 12th week of pregnancy.
abdominal circumference, which measures 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, which measures the distance around the baby's head. It is a way to track growth.
foetal limb measurement, which checks the length of the baby's femur, which is the long leg bone. It correctly predicts the due date within 7 days.
transcerebellar diametres, which measure the distance around one part of a baby's brain.
mean ultrasound age, which averages all the above measurements to arrive at a reasonable due date.
Ultrasound measurements that are used less often include:
length of the lower jaw bone
foot length
inner and outer orbital, or eye distance
chest diametre
liver size
length of the collar bone
thigh diametre
Accurate dating methods offer a woman a good estimate of the day her baby will arrive. Ultrasound has other benefits, too. With it, foetal growth can be monitored and many birth defects and health problems can be ruled out. That helps pave the way for a successful birth.
Author: Eva Martin, MD Reviewer: HealthAnswers Australia Medical Review Panel Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia Last Updated: 1/10/2001 Contributors Potential conflict of interest information for reviewers available on request
