Which of the following procedure is involved in injecting a small flexible plastic tube into the uterus through the vagina?

The correct option is Chorionic villi sampling.

To understand why this is the correct answer, let's break down the procedure and compare it with the other options provided:

1. Understanding Chorionic Villi Sampling (CVS):
Chorionic villi sampling is a prenatal test used to detect genetic abnormalities and chromosomal disorders in a fetus. During a transcervical CVS procedure, a clinician inserts a thin, flexible plastic tube (catheter) through the vagina and cervix into the uterus. Using ultrasound guidance, this tube is directed to the placenta to gently suction a small sample of chorionic villi (tiny finger-like projections of placental tissue) for laboratory analysis.

2. Comparing with Other Options:
- Amniocentesis: This procedure involves inserting a long, thin needle through the mother's abdominal wall directly into the amniotic sac inside the uterus to withdraw amniotic fluid. It does not involve inserting a flexible plastic tube through the vagina.
- Alpha-fetoprotein screening: This is a simple blood test performed on a sample of the pregnant woman's blood. It is non-invasive and does not involve any vaginal or uterine instrumentation.
- None of these: Since chorionic villi sampling matches the described technique, this option is incorrect.

Therefore, the procedure that specifically involves injecting or inserting a small, flexible plastic tube into the uterus through the vagina is indeed Chorionic villi sampling.