**Core Concept**
Fetal kidney development is a critical process, and understanding when and how fetal kidneys start producing urine is essential for neonatal medicine and obstetrics. The development of the fetal kidney involves the formation of the metanephric blastema, the ureteric bud, and the subsequent differentiation of nephrons. This process is regulated by a complex interplay of genetic and environmental factors.

**Why the Correct Answer is Right**
The fetal kidneys start producing urine by approximately 12 weeks of gestation. This is due to the differentiation of the metanephric blastema into functioning nephrons, which are responsible for filtering waste products from the blood and producing urine. The presence of functioning nephrons allows for the production of amniotic fluid, which is essential for fetal lung development and proper fetal growth. The kidneys also produce antidiuretic hormone (ADH)-independent urine, which is crucial for fetal development.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the fetal kidneys do not start producing urine at this gestational age. The development of the fetal kidney is a complex process that involves multiple stages, and the initial production of urine occurs significantly earlier than this option suggests.
**Option B:** This option is incorrect because it does not specify a gestational age, making it an ambiguous and unreliable answer choice. The correct gestational age for the onset of fetal kidney urine production is a critical piece of information that cannot be ignored.
**Option C:** This option is incorrect because it implies that the fetal kidneys do not start producing urine until after birth. This is not accurate, as the fetal kidneys are capable of producing urine by approximately 12 weeks of gestation.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the fetal kidneys start producing urine by approximately 12 weeks of gestation, which is a critical milestone in fetal development. Understanding this process is crucial for diagnosing and managing fetal renal dysfunction and other related complications.

**Correct Answer:** C. 12 weeks