**Core Concept**
A threatened abortion refers to a condition where there is vaginal bleeding during pregnancy, without cervical dilatation or passage of products of conception. This condition necessitates careful evaluation and monitoring to assess the risk of miscarriage and potential complications. The underlying concern is the integrity and viability of the embryo, as well as the presence of any underlying causes for the bleeding.

**Why the Correct Answer is Right**
The presence of threatened abortion is associated with an increased risk of pregnancy-induced hypertension (PIH), also known as gestational hypertension. This is because threatened abortion often involves some degree of placental abruption, where the placenta separates from the uterine wall. Placental abruption can disrupt the normal functioning of the placenta, leading to decreased blood flow to the fetus and increased blood pressure in the mother. The exact mechanism involves the release of various vasoconstrictive substances, such as thromboxane A2, which cause blood vessels to constrict and lead to hypertension.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not directly related to the increased risk of pregnancy complications following threatened abortion.
* **Option B:** While pre-existing hypertension can increase the risk of pregnancy complications, it is not a direct consequence of threatened abortion.
* **Option C:** This option is not specifically related to the increased risk of pregnancy complications following threatened abortion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that threatened abortion is a potential marker for underlying placental dysfunction and increased risk of pregnancy-induced hypertension. Close monitoring of blood pressure and fetal well-being is crucial in these cases.

**Correct Answer:** D. Pregnancy-induced hypertension.