Abstract
Acetylcholinesterase test of amniotic fluid is commonly used as a diagnostic indicator of fetal neural tube defect in singleton pregnancies. Because of its large size it is unlikely that the acetylcholinesterase molecule diffuses across fetal membranes (1) and the test would hence also be useful in twin pregnancy if one of the fetuses were found to have a neural tube defect and the other had a normal sonographic appearance. Unfortunately this has proved to be incorrect.