What should pregnant individuals avoid doing from the first trimester onward during exercise?

The correct answer is that pregnant individuals should avoid the supine position during exercise starting from the first trimester. This recommendation is based on physiological changes that occur during pregnancy. As the uterus expands, lying flat on the back can potentially compress the inferior vena cava, a major vein that returns blood to the heart. This compression can lead to decreased blood flow to the uterus and may result in feelings of dizziness, decreased blood pressure, and compromised fetal circulation. Therefore, it is advisable for pregnant individuals to modify their exercise routines to avoid positions that could create this scenario, especially as the pregnancy progresses.

The other options, while they may pose considerations during pregnancy, do not carry the same direct implications for safety during exercise. Exercising below sea level and high-altitude training might present challenges related to oxygen availability, but they are not typically contraindicated for all pregnant individuals without further context. Weight-bearing activities can be beneficial and are generally encouraged for maintaining bone density and fitness during pregnancy, as long as they are performed with caution and appropriate modification.