if measurement of neural tube is greater than 2.5mm indicates down syndrome

Neural tube.

The neural tube closes around the 28th day of gestation, so that would be at the end of the fourth week.

Nervous Tissue

In the embryo the neural tube forms the central nervous system in the fetus, which is the brain and spinal cord.

The reason pregnant women are given folic acid supplementation is to help prevent neural tube defects, which can range from fairly benign all the way to anencephaly (where the baby is born without a brain).