**Core Concept**
Amniocentesis is a prenatal diagnostic procedure used to detect chromosomal anomalies and other fetal abnormalities. It involves the withdrawal of a small sample of amniotic fluid from the uterus to analyze for genetic disorders, infections, and other conditions. The indications for amniocentesis are based on specific risk factors that warrant further investigation.

**Why the Correct Answer is Right**
Gestation diabetes, also known as gestational diabetes mellitus (GDM), is a condition characterized by high blood sugar levels during pregnancy. While GDM can increase the risk of fetal macrosomia and other complications, it is not an indication for amniocentesis for chromosomal anomaly detection. The American College of Obstetricians and Gynecologists (ACOG) recommends amniocentesis for women with a high risk of chromosomal abnormalities, such as those over 35 years old, with a previous history of Down's syndrome, or with a family history of chromosomal anomalies.

**Why Each Wrong Option is Incorrect**
**Option B:** Previous Down's child is a valid indication for amniocentesis, as it increases the risk of chromosomal abnormalities in future pregnancies.

**Option C:** Maternal age more than 35 is a well-established risk factor for chromosomal abnormalities, including Down's syndrome. Amniocentesis is often recommended for women in this age group.

**Option D:** Parents with chromosomal anomaly can pass on genetic disorders to their offspring, making it a valid indication for amniocentesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that amniocentesis carries a small risk of complications, such as miscarriage and fetal infection. Therefore, it's typically recommended for women with a high risk of chromosomal abnormalities, rather than those with lower-risk conditions like gestational diabetes.

**✓ Correct Answer: A. Gestation diabetes**