The possibility of prenatal screening for genetic disorders was raised as early as the mid-1950s, and with the introduction in 1966 of amniocentesis for sampling fetal material, it became possible to identify pregnancies with trisomy 21 (Down syndrome), the most common prenatal genetic abnormality. The fetal cells in the amniotic fluid could be cultured, then harvested, followed by chromosome spreading on microscope slides. These chromosome spreads, each representing the chromosomes from a single cell nucleus, could be stained, visualised by light microscopy and counted to establish the chromosome number. However, diagnosis of Down syndrome was expensive, and in the early days of amniocentesis, there was an associated risk of miscarriage; most countries therefore recommended this procedure only for women who were identified as having a raised risk of chromosome abnormality. As it is well established that raised maternal age increases the risk of Down syndrome, amniocentesis was first offered only to women above an age cut-off (usually 35). However, although the risk to an individual woman of having a Down syndrome pregnancy is greater in this age group, the majority of Down syndrome babies are born to younger women, due to the preponderance of pregnancies in the younger group.