**Core Concept**
The development of primary teeth involves a complex interplay of cellular differentiation, growth, and calcification. Calcification, the process of mineralization, is a critical step in the formation of the tooth structure.

**Why the Correct Answer is Right**
The calcification of primary mandibular first molars begins around the age of 4-5 months in utero, as part of the primary tooth development process. This process is initiated by the proliferation of ameloblasts, which secrete an organic matrix that serves as a scaffold for mineral deposition. The mineralization of the tooth is facilitated by the presence of amelogenins, which are proteins that regulate the formation of hydroxyapatite crystals. The calcification process continues until the tooth is fully formed and erupted into the oral cavity.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the calcification of primary mandibular first molars begins before birth, not after birth.
**Option B:** This is incorrect because the calcification of primary mandibular first molars does not occur at the same time as the calcification of primary incisors.
**Option C:** This is incorrect because the calcification of primary mandibular first molars does not occur at the same time as the calcification of primary canines.

**Clinical Pearl / High-Yield Fact**
It's essential for dental and medical professionals to be aware of the developmental timeline of primary teeth, as abnormalities in tooth development can be indicative of systemic or genetic disorders.

**Correct Answer:** D.