What is the cause of myometrial contraction in the second trimester pregnancy?

Basically your fetus is growing now and putting on weight. Your blood volume increases due to the baby and you may notice that your blood pressure is lower than normal.

Trimester= three simesters. If pregnancy lasts nine months, common sense would lead me to believe the 2nd trimester is three months.

The second trimester is weeks 13-28, where the baby does most growing, so the weight at the beginning and the end is very different

Weeks 13-28 of pregnancy

It begins when your baby is twelve weeks old, which means you will be in your 13th week of pregnancy.