**Core Concept:** Nitabuch's membrane is a thin, transparent membrane that develops on the surface of the lens during pregnancy, particularly in the third trimester. It is composed of hyaluronic acid, chondroitin sulfate, and other extracellular matrix proteins. The membrane assists in maintaining lens transparency during pregnancy and facilitates the passage of oxygen to the fetus.

**Why the Correct Answer is Right:** Nitabuch's membrane is essential for fetal oxygenation and lens transparency. It serves as a protective barrier between the lens and the amniotic fluid. In the absence of this membrane, the lens may become opaque due to the lack of oxygenation, leading to congenital cataracts and vision impairment in the newborn. This can be a significant clinical problem, as cataracts can cause visual impairment or blindness if left untreated.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because Nitabuch's membrane is present in the majority of cases. It is essential for fetal wellbeing and prevents lens opacification in the majority of pregnancies.
B. Although Nitabuch's membrane can be present in the first trimester, it is not absent here. The membrane plays a crucial role throughout pregnancy, ensuring fetal oxygenation, lens transparency, and preventing congenital cataracts.
C. This option is incorrect because Nitabuch's membrane is not absent in the second trimester. Its presence is essential for fetal wellbeing and maintaining lens transparency in this stage of pregnancy.
D. Similarly, Nitabuch's membrane is not absent in the third trimester. Its presence helps maintain lens transparency and oxygenation, preventing congenital cataracts in the newborn.

**Clinical Pearl:** Oral corticosteroids administered to pregnant women can interfere with the development of Nitabuch's membrane, increasing the risk of congenital cataracts. This highlights the importance of considering potential medication side effects during pregnancy and monitoring fetal wellbeing closely.