'midterm' means: in the middle of a period of time.
Barack Obama is President for 4 years. The midterm elections are the elections at the end of his second year: November 2010.
Midterm exams are exams in the middle of the school year. The school year is from September to June; the midterm exams are at the end of December.
Midterm pregnancy means in the middle trimester of the pregnancy. 'Trimester' means a period of 3 months. The first trimester is months 1, 2 and 3. The middle trimester is months 4, 5 and 6. The last trimester is months 7, 8 and 9.
