The fluid in the amniotic cavity, or gestational sac, which contains cells derived from the fetus. Sampled with amniocentesis.

A form of albumin (a protein in the blood) produced only by the fetus, but crosses the placenta and so is detectable in the mother's blood (measured with a serum alpha fetoprotein, as well as being usefully measured in amniotic fluid obtained by amniocentesis. Detectable in higher than usual concentration with certain open abnormalities involving the fetus's brain and spinal cord (namely anencephaly and spina bifida). Present in lower than usual concentration when there is a trisomyin the fetus, such as Down syndrome (or trisomy 21), trisomy 18 or possibly Klinefelter syndrome -- screening for all of which can be done with a triple test.

Sampling the fluid from the amniotic cavity after about 14 or 15 weeks of pregnancy. Cells from the fetus can be set up in culture for a karyotype, or for special diagnoses can be examined more quickly by fluorescent in situ hybridisation (FISH), comparative genomic hybridisation (CGH) or by polymerase chain reaction (PCR). Other substances in the amniotic fluid (such as alpha fetoprotein, or AFP) can be measured to indicate whether the fetus is normal or not.

Measurement of alpha fetoprotein in blood serum. High levels can indicate a birth defect involving the brain or spinal cord, such as anencephaly or spina bifida (confirmed if amniocentesis shows high AFP levels in the amniotic fluid). Low levels can indicate an increased risk of Down syndrome (or trisomy 21), trisomy 18 and, occasionally, Klinefelter syndrome, signaling the need for a karyotype of the fetus's tissues by chorionic villus sampling (CVS) or amniocentesis.