Chromosome Analysis, Amniotic Fluid

Amniotic fluid, fetal urine, or cystic hygroma fluid

20 to 30 mL (15 to 20 mL for early amniocentesis)

5 mL (minimum volume may delay results due to fewer cells available for culture)

Sterile plastic conical tube

The patient preferably should have had ultrasound studies (to verify fetal life, detect multiple gestation, confirm gestational age, localize fetus/placenta).

The test determines fetal karyotype. The test allows prenatal detection of chromosomal rearrangements, aneuploidy, or mosaicism. Such groups include women who:

• are age 35 years of age or older

• have a previous child having chromosome abnormality or multiple congenital abnormalities

• have had two or more previous spontaneous abortions

• have a family history of a chromosome abnormality

• are known carriers of an X-linked disorder

• are 31 years of age or older with twin pregnancies

• have abnormal fetal ultrasound findings

• have a positive maternal serum marker screen

Additional biochemical or molecular tests may be performed on the cultured amniocytes.

Although the overall culture success rate is reported as >99%, culture failure can result. Reasons include but are not limited to lack of amniocytes in the fluid, and contamination of the fluid with bacteria or yeast.

In situ cell culturing of amniocytes to investigate numerical and/or structural chromosome abnormalities

Fetal loss rate at 14 to 18 week sampling is considered to be 0.5%, and 2% to 3% at 10 to 13 weeks.¹ Chorionic villus sampling (CVS) may be safer than early amniocentesis for early prenatal diagnosis of cytogenetic abnormalities.^2,3 The risk of miscarriage with CVS is 1% to 1.5% but the risk of maternal infection appears to be higher with CVS than with amniocentesis.⁴ Cytogenetic analyses using such samples allow for an early gestational testing based on a 10- to 11-week placental biopsy and a five-day cytogenetic study. Most failures are due to an inappropriate biopsy containing only maternal decidua. Chromosomal aberrations were found in 4.6% of fetuses in women older than 38 to 40 years of age. Trisomy 21 was the most common abnormality (62%). Klinefelter syndrome (11%) and Edwards syndrome, trisomy 18 (11%) were next most frequent in the cases of advanced maternal age.

Prenatal diagnosis is possible for more than 1000 inherited diseases. Most are inherited in an autosomal recessive manner. Antenatal diagnosis using gene probes has become available for cystic fibrosis, muscular dystrophy, sickle cell anemia, hemophilia, and many other genetic abnormalities. This can be done from either cultured amniotic fluid cells or chorionic villous sampling.⁵