Precautions to be taken before you Start -
Warm up first- Always start with 5-10 min warm up like light walking before you start stretching because when muscles are cold during stretch it can tear up the muscle if not done properly.
Gradual stretching - Always start slow and gradually increase the stretch , hold the stretch for at least 30-40 seconds.
No overdoing - flexibility is achieved through regular, diligent practice. It takes weeks or months to safely increase your flexibility. Going too far, too fast may result in injuries. Lessen your stretch or stop stretching if you notice sensations signifying that you have gone too deep into your stretch, such as sharp pain in the muscles. Stop stretching immediately if there is pain in the joints, such as the knees or elbows. Signs of starting to go too deep may include holding your breath or clenching your jaw.
Type of Pain- stretching should provide a comfortable mild ache type of pain if its sharply painful, intolerable , is making muscles sore then immediately stop the stretch.
Type of stretch- stretching should not include bouncing or jerky movements .
Breathing - Always breath normally , do not hold your breath it can cause a drop in blood pressure . Here is a video which will give you more insight on basic stretches during pregnancy , for more details post your comments.