**Core Concept**
Post-term labour refers to the onset of labour after 42 weeks of gestation, exceeding the normal gestational period. This condition is associated with an increased risk of fetal compromise and adverse pregnancy outcomes. The exact cause of post-term labour is multifactorial, involving hormonal, uterine, and fetal factors.

**Why the Correct Answer is Right**
Post-term labour is often linked to a decrease in the levels of human chorionic gonadotropin (hCG) and an increase in the levels of relaxin, which contribute to uterine relaxation and decreased uterine tone. Additionally, post-term pregnancies may be associated with an increased risk of placental insufficiency, leading to fetal distress and acidosis. The exact mechanisms underlying post-term labour are complex and involve interactions between the mother, placenta, and fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specifically relate to the definition of post-term labour. While prolonged pregnancy can be associated with various complications, this option does not accurately describe post-term labour.

**Option B:** This option is incorrect as it is more commonly associated with preterm labour, which occurs before 37 weeks of gestation. While both preterm and post-term labour involve an abnormal duration of pregnancy, they have distinct underlying causes and consequences.

**Option C:** This option is incorrect as it is associated with a different condition, typically characterized by an abnormal uterine contraction pattern and often managed with tocolytics to delay preterm birth.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between post-term and prolonged pregnancy, as the management strategies and outcomes differ significantly between these conditions. Post-term labour requires prompt recognition and intervention to minimize fetal risks.

**Correct Answer: D. Post-term labour is seen in prolonged pregnancy beyond 42 weeks of gestation.**