Amnio

Measuring alpha-fetoprotein levels to detect conditions like spina bifida.

According to WebMD , the fluid is dynamic and changes as the pregnancy progresses:

In late pregnancy, it can be tested for surfactant levels to assess if the baby's lungs are developed enough for delivery.

Mostly water and electrolytes derived from maternal plasma.

Becomes a mixture of fetal urine, lung secretions, and exfoliated fetal cells.

Testing fetal cells for chromosomal abnormalities like Down syndrome or cystic fibrosis.