**Question:** Fetal sex can be detected by USG at:

**Core Concept:** Transvaginal ultrasound (TVUS) is a technique that allows visualizing the fetus within the uterus and determining various aspects of fetal development. In this question, we are assessing the ability to identify the sex of the fetus using ultrasound.

**Why the Correct Answer is Right:** The correct answer refers to the transvaginal ultrasound (TVUS) approach, which involves inserting a transducer into the vagina to obtain a clearer image of the fetus. At around 18-22 weeks of gestation, the fetal genitalia begin to differentiate, allowing for accurate sex determination. The human fetus undergoes significant organ development during this period, including the formation of the penis, testes, and vagina, which facilitates the differentiation of male and female genitalia.

**Why Each Wrong Option is Incorrect:**

A. Early in pregnancy (before 18 weeks): At this stage, the fetal genitalia are too small and not yet differentiated, making sex determination unreliable.

B. Late in pregnancy (after 36 weeks): The fetus is too large, and the organs may not be fully developed, leading to inaccurate assessment of fetal sex.

C. Transabdominal ultrasound (TAUS): This method uses a transducer placed on the abdomen, which provides less accurate results than transvaginal ultrasound due to the limited resolution and the presence of the uterus, bladder, and other abdominal organs.

D. Around 7-9 weeks: At this stage, the fetal genitalia are too small and not yet differentiated, making sex determination unreliable.

**Clinical Pearl:** Transvaginal ultrasound provides a clearer image of the fetus, contributing to more accurate fetal sex determination during the appropriate gestational age window (18-22 weeks). This method is essential for accurate sex determination and prenatal sex-selective procedures, if needed.

**Correct Answer:** D. Around 7-9 weeks

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Transvaginal ultrasound (TVUS), performed around 7-9 weeks, is the most accurate method for determining fetal sex due to the clearer image obtained from inserting the transducer through the vagina. The fetal genitalia can be visualized earlier, allowing for accurate sex determination earlier in pregnancy. This method is crucial for prenatal sex-selective procedures, as well as for ensuring accurate counseling and genetic counseling for couples considering gender-selective termination.