Stages of pregnancy
First trimester (week 1-week 12)
At four to five weeks
» Your baby's brain and spinal cord have begun to form.
» The heart begins to form.
» Arm and leg buds appear.
» Your baby is now an embryo and one-twenty-fifth inch long.
At eight weeks
» All major organs and external body structures have begun to form.
» Your baby's heart beats with a regular rhythm.
» The arms and legs grow longer, and fingers and toes have begun to form.
» The sex organs begin to form.
» The eyes have moved forward on the face and eyelids have formed.
» The umbilical cord is clearly visible.
» At the end of eight weeks, your baby is a fetus and looks more like a human. Your baby is nearly 1 inch long and weighs less than one-eighth ounce.
At 12 weeks
» The nerves and muscles begin to work together. Your baby can make a fist.
» The external sex organs show if your baby is a boy or girl. A woman who has an ultrasound in the second trimester or later might be able to find out the baby's sex.
» Eyelids close to protect the developing eyes. They will not open again until the 28th week.
» Head growth has slowed, and your baby is much longer. Now, at about 3 inches long, your baby weighs almost an ounce.
Second trimester (week 13-week 28)
At 16 weeks
» Muscle tissue and bone continue to form, creating a more complete skeleton.
» Skin begins to form. You can nearly see through it.
» Meconium (mih-KOH-nee-uhm) develops in your baby's intestinal tract. This will be your baby's first bowel movement.
» Your baby makes sucking motions with the mouth (sucking reflex).
» Your baby reaches a length of about 4 to 5 inches and weighs almost 3 ounces.
At 20 weeks
» Your baby is more active. You might feel slight fluttering.
» Your baby is covered by fine, downy hair called lanugo (luh-NOO-goh) and a waxy coating called vernix. This protects the forming skin underneath.
» Eyebrows, eyelashes, fingernails, and toenails have formed. Your baby can even scratch itself.
» Your baby can hear and swallow.
» Now halfway through your pregnancy, your baby is about 6 inches long and weighs about 9 ounces.
At 24 weeks
» Bone marrow begins to make blood cells.
» Taste buds form on your baby's tongue.
» Footprints and fingerprints have formed.
» Real hair begins to grow on your baby's head.
» The lungs are formed, but do not work.
» The hand and startle reflex develop.
» Your baby sleeps and wakes regularly.
» If your baby is a boy, his testicles begin to move from the abdomen into the scrotum. If your baby is a girl, her uterus and ovaries are in place, and a lifetime supply of eggs have formed in the ovaries.
» Your baby stores fat and has gained quite a bit of weight. Now at about 12 inches long, your baby weighs about 1½ pounds.
Third trimester (week 29-week 40)
At 32 weeks
» Your baby's bones are fully formed, but still soft.
» Your baby's kicks and jabs are forceful.
» The eyes can open and close and sense changes in light.
» Lungs are not fully formed, but practice "breathing" movements occur.
» Your baby's body begins to store vital minerals, such as iron and calcium.
» Lanugo begins to fall off.
» Your baby is gaining weight quickly, about one-half pound a week. Now, your baby is about 15 to 17 inches long and weighs about 4 to 4½ pounds.
At 36 weeks
» The protective waxy coating called vernix gets thicker.
» Body fat increases. Your baby is getting bigger and bigger and has less space to move around. Movements are less forceful, but you will feel stretches and wiggles.
» Your baby is about 16 to 19 inches long and weighs about 6 to 6½ pounds.
Weeks 37–40
» At 39 weeks, your baby is considered full-term. Your baby's organs are ready to function on their own.
» As you near your due date, your baby may turn into a head-down position for birth. Most babies "present" head down.
» At birth, your baby may weigh somewhere between 6 pounds 2 ounces and 9 pounds 2 ounces and be 19 to 21 inches long. Most full-term babies fall within these ranges. But healthy babies come in many different sizes.
Source
The Office on Women's Health
https://www.womenshealth.gov/pregnancy/youre-pregnant-now-what/stages-pregnancy