**Core Concept**
Tuberculosis (TB) in pregnancy poses significant risks to both the mother and the fetus. The disease can reactivate in a pregnant woman, leading to a flare-up. This reactivation is a critical consideration in the management of TB in pregnancy.

**Why the Correct Answer is Right**
TB typically flares up in the second trimester of pregnancy, which is around 20-24 weeks. This period is characterized by the physiological changes that occur during pregnancy, such as increased blood volume, cardiac output, and metabolic changes, which can lead to the reactivation of latent TB. The reactivation of TB in the second trimester is thought to be due to the increased levels of cortisol, estrogen, and progesterone, which can suppress the immune system and lead to the reactivation of latent TB.

**Why Each Wrong Option is Incorrect**
* **Option A:** TB flare-up in the first trimester is less common due to the immune-modulating effects of estrogen and progesterone, which are higher during this period.
* **Option B:** TB flare-up in the third trimester is possible, but it is less common than in the second trimester due to the changes in the immune system and the increased levels of cortisol.
* **Option D:** TB flare-up in the postpartum period is a possibility, but it is not the most common period for TB flare-up in pregnancy.

**Clinical Pearl / High-Yield Fact**
The reactivation of TB in the second trimester is a critical consideration in the management of TB in pregnancy, and it highlights the importance of screening for latent TB in all pregnant women, especially those with a history of TB exposure or those from high-prevalence areas.

**Correct Answer:** C.