This middle section of pregnancy is often thought of as the best part of the experience. By this time, any morning sickness is probably gone and the discomfort of early pregnancy has faded. The baby will start to develop facial features during this month. You may also start to feel movement as your baby flips and turns in the uterus. During this trimester, many people find out the sex of the baby. This is typically done during an anatomy scan (an ultrasound that checks your baby’s physical development) around 20 weeks.
Month 4 (weeks 13 through 16)
Your baby's heartbeat may now be audible through an instrument called a doppler. The fingers and toes are well-defined. Eyelids, eyebrows, eyelashes, nails and hair are formed. Teeth and bones become denser. Your baby can even suck his or her thumb, yawn, stretch and make faces.
The nervous system is starting to function. The reproductive organs and genitalia are now fully developed, and your doctor can see on ultrasound if you are having a boy or a girl.
By the end of the fourth month, your baby is about 15 cm long and weighs about 0.11 kg.
Month 5 (weeks 17 through 20)
At this stage, you may begin to feel your baby moving around. Your baby is developing muscles and exercising them. This first movement is called quickening and can feel like a flutter.
Hair begins to grow on baby's head. Your baby's shoulders, back and temples are covered by a soft fine hair called lanugo. This hair protects your baby and is usually shed at the end of the baby's first week of life.
The baby's skin is covered with a whitish coating called vernix caseosa. This "cheesy" substance is thought to protect your baby's skin from the long exposure to the amniotic fluid. This coating is shed just before birth.
By the end of the fifth month, your baby is about 25 cm long and weighs from 0.25 to 0.5 kg.
Month 6 (weeks 21 through 24)
If you could look inside the uterus at your baby right now, you would see that your baby's skin is reddish in color, wrinkled and veins are visible through the baby's translucent skin. Baby's finger and toe prints are visible. In this stage, the eyelids begin to part and the eyes open.
Baby responds to sounds by moving or increasing the pulse. You may notice jerking motions if baby hiccups.
If born prematurely, your baby may survive after the 23rd week in some parts of the world with intensive care.
By the end of the sixth month, your baby is about 30 cm long and weighs about 1 kg.
Month 7 (weeks 25 through 28)
Your baby will continue to mature and develop reserves of body fat. At this point, the baby's hearing is fully developed. The baby changes position frequently and responds to stimuli, including sound, pain and light. The amniotic fluid begins to diminish.
If born prematurely, your baby would be likely to survive after the seventh month in most parts of the world.
At the end of the seventh month, your baby is about 36 cm long and weighs from 1.5 to 2 kg.
Reference from @Cleveland Clinic