In the second trimester, it is normal to feel extremely horny because there is more blood flowing to the genitals. The first trimester is usually full of morning sickness and fatigue and that might not put a woman in the mood for sex. During the last trimester, be careful to prevent preterm labour.
It is essential to have as much sex as you can before the baby comes because the decline in your sex life when the baby arrives would be drastic and abrupt.