Establishment of fetoplacental circulation seen at
**Core Concept**
The establishment of fetoplacental circulation is a critical developmental process in pregnancy, marking the transition from the embryonic circulation to a functional placental circulation that supports fetal growth and development.
**Why the Correct Answer is Right**
The correct answer is associated with the formation of the umbilical cord and the establishment of blood flow between the fetus and the placenta. This process begins around 12-13 days post-fertilization, when the allantois and the chorion fuse to form the umbilical cord. The **allantoic vessels** give rise to the **umbilical arteries** and veins, which then connect to the placenta, establishing a functional fetoplacental circulation.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the establishment of fetoplacental circulation occurs before the completion of the 4th week of gestation, not around the 4th month.
**Option B:** This option is incorrect because the formation of the umbilical cord is a prerequisite for the establishment of fetoplacental circulation, which does not occur simultaneously with the development of the kidneys.
**Option C:** This option is incorrect because the establishment of fetoplacental circulation is a separate process from the development of the lungs, which occurs later in gestation.
**Clinical Pearl / High-Yield Fact**
The establishment of fetoplacental circulation is a critical process that ensures the exchange of oxygen, nutrients, and waste products between the fetus and the mother, supporting fetal growth and development.
**Correct Answer: C. 18-20 days post-fertilization.**