Of course, you must first know your due date to calculate these figures. Doctors use various methods to determine the gestational age of a pregnancy. The most common test is the ultrasound, which measures the size of the gestational sac and the length between the crown and rump to determine the age of the fetus in weeks. Other common methods of determining gestational age include last menstrual period, bimanual exam and serum hCG testing.
An ultrasound uses sound waves passed through the stomach and reflected back to the ultrasound machine to create a physical, live picture of the fetus. During an ultrasound, women may notice the doctor or ultrasound technician placing several markers on the screen. These markers are used to measure the gestational sac and the length between the crown (head) and rump. If the ultrasound is performed after the 6th-week gestation, the length of the fetus from crown to rump is added to 6 weeks to determine gestational age. For instance, if the crown-rump measurement is 21 mm, the gestational age of the fetus is 9 weeks. (6 weeks plus 21 days = 9 weeks.) If an ultrasound is performed before the 6th-week gestation, age is determined based on other factors, including the size of the fetus and development of the gestational sac. Gestational sac measurements are highly accurate from 4 weeks gestation.
Last Menstrual Period
Using the first day of your last menstrual cycle, gestational age can be determined (plus or minus 2 weeks.) The doctor adds 280 days to the first day of your last menstrual cycle to determine a due date. Based on that number and today’s date, gestational age is estimated. This number is not always accurate, but it is a good indication of fetal size.
During a bimanual exam, the gynecologist or obstetrician will place two gloved fingers into the vagina and press on the cervix. The other hand is used to press down on the uterus. The size, shape, and firmness of the uterus are used to determine pregnancy and, possibly, gestational age.
Serum HCG Testing
hCG or human chorionic gonadotropin is found in the blood of all pregnant women. Serum hCG testing is often completed as part of the birth defect screening tests done between weeks 15 and 20 of gestation, but in some cases obstetricians and gynecologists require a pregnancy blood test before scheduling the first prenatal appointment.
During the first three weeks of gestation, serum HCG levels range from 5 to 50, with average levels measuring about 14, 21 and 42 miu/ml, respectively. Week 4 levels range from 10 to 425 and week 5 levels range from 19 to 7,340. From there, hCG levels continue to rise until they peak around the 12th week at around 288,000 miu/ml.
The broad range of overlapping hCG levels can make it hard to determine accurate gestational age, but the test is highly effective at determining multiple pregnancies. More than one fetus developing in the body means higher than normal hCG levels, which could spark further testing, including an ultrasound to rule out multiple pregnancies.