Ans. (c) Nuchal translucency Ref.: D.C. Dutta 8th ed. / 735 * Nuchal translucency is a fluid behind the neck of the fetus. It is an anechoic space visible and seen in all fetuses between 11-14 weeks of age * The most common cause of increased nuchal translucency(> 3 mm) is aneuploidy where TURNER syndrome is the most common cause followed second by Down syndrome. * The reason why it is seen in 11-14 weeks is because at that time fetal lymphatics are developing and peripheral resistance is high and gradually disappears. * Remember: Cystic hygroma which is seen in Turner syndrome is high incidence is due to defective development of lymphatics.