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Alpha fetoprotein Testing
As your baby grows inside you, it produces alpha-fetoprotein. This protein is found in increasing amounts in the amniotic fluid. Some alpha-fetoprotein crosses fetal membranes and enters your circulation. It is possible to measure the amount of alpha-fetoprotein by drawing your blood. The level of this protein can be meaningful during pregnancy. An alpha-fetoprotein (AFP) test is usually done between 16 and 18 weeks of gestation. The timing of the test is important and must be correlated to the gestational age of your pregnancy and to your weight.

An elevated level of alpha-fetoprotein can indicate problems with the fetus, such as spina bifida (spinal-cord problem) or anencephaly (serious central-nervous-system defect). Some researchers have even found an association between a low level of alpha-fetoprotein and Down syndrome. In the past, amniocentesis was the only way to test for Down syndrome.
If the level of alpha-fetoprotein is abnormal, a careful ultrasound examination is done to look for spina bifida, anencephaly and Down syndrome. This ultrasound may help determine how far along in pregnancy you are.

This test is not done on all pregnant women, although it is required in some states. It is not used routinely in Canada. If the test isn't offered to you, ask about it. There is relatively little risk to you, and it tells your caregiver how your fetus is growing and developing.