Yes, it is possible to feel Braxton Hicks contractions during the 12th week of a second pregnancy. These contractions are often described as stomach tightening and can occur once or twice a day. While they may cause some discomfort, they are usually not painful. Braxton Hicks contractions are considered normal and are your body's way of preparing for labor. They can be more noticeable in subsequent pregnancies due to increased sensitivity in the uterus. However, if you experience severe pain, bleeding, or any other concerning symptoms along with these contractions, it is important to consult your healthcare provider for further evaluation.