A baby grows and changes a lot in the 9 months of pregnancy. It can be helpful to know how your baby changes from month to month during this time.
One of the things it can be helpful to remember is that the months of pregnancy do not exactly match the months of the calendar year. The 9 months of pregnancy are really the 40 weeks from the first day of your last menstrual period (if that date is known).
About 5 to 7 days after the sperm fertilizes the egg (conception), the egg attaches to the lining of the uterus. This is called implantation. The fertilized egg then begins to grow in the uterus. At this time the baby is called an embryo.
Shortly after implantation the placenta and umbilical cord begin to form. The placenta and umbilical cord give food and oxygen to your baby and carry away the baby's wastes. Your baby is enclosed in a sac of fluid that protects the baby from bumps and pressure.
In another week the baby has a spinal cord and starts developing the bones in the spine. The heart also forms, and it begins to beat on the 25th day after conception (5 to 6 weeks after the last menstrual period). By the end of the first 6 weeks of pregnancy, your baby has a head and trunk. The eyes, brain, mouth, inner ears, and digestive system begin to form, as do tiny buds that will be the arms and legs.
By the end of 6 weeks your baby is about a half inch long (1 centimeter) and weighs a fraction of an ounce (or a few grams).
The second month is especially important in the development of your baby. Drugs, viruses, or environmental factors, such as pesticides, could affect the baby’s development at this time and cause birth defects.
Your baby's development is very fast during the second month. By the end of the second month, all of your baby's major body organs and body systems, including the lungs, liver, and stomach, have started developing. Eyelids form and grow but are sealed shut, and ears are formed. Ankles, toes, wrists, fingers, and sexual organs develop.
At the end of the second month, the baby is a little over 1 inch long (3 centimeters) and still weighs less than 1 ounce (28 grams). The baby is now in a sac called the amniotic sac. The baby’s head is about the size of the rest of its body.
By the end of the third month, your baby is completely formed. During this month the fingers and toes have soft nails, and tooth buds have formed under the gums. Hair may start to appear on the head, and the head is large compared with the rest of the body. Swellings on the side of the neck show where the ears will be and the inner ears are forming. There is a bit of a nose and the lips are forming. The kidneys develop and start draining urine into the bladder. The baby's heart has four chambers and beats at 120 to 160 beats per minute. The baby may open and close its mouth and start moving its hands, legs, and head. At this point, however, you will not feel this movement.
Your baby is now 4 inches long (10 centimeters) and weighs just a little over 1 ounce (over 28 grams). This is the end of the first trimester of pregnancy. The baby is now called a fetus.
The baby now has skin that is pink and somewhat transparent, and eyebrows and eyelashes begin to appear. The baby's body is covered in fine soft hair called lanugo. The baby is storing fat under the skin, and the bones are making blood cells. The external sex organs have become distinctly male or female. Your baby has vocal cords, taste buds, and a strong heartbeat. Now the body grows faster than the head.
The baby moves, kicks, sleeps, wakes, swallows, and passes urine. You may start to feel a slight sensation of movement in your lower belly. This feeling is like bubbles or fluttering. You may first feel the baby's movement (called quickening) at this time.
Your baby is now 6 to 7 inches (16 to 18 centimeters) long and weighs about 6 ounces (about 170 grams).
Your baby will have a real growth spurt this month. The internal organs are maturing, and the fingernails have grown to the tips of the fingers. The number of nerve cells in the brain increases rapidly this month. Stool (called meconium) begins to form in the intestines. The baby now has regular times that it sleeps and is awake and moving. The baby may suck its thumb. You will find that your baby is much more active, turning from side to side and head over heels. The baby drinks fluid and urinates into the amniotic sac. The amniotic fluid transfers material in the sac to the blood in the mother's circulation. The amniotic fluid is completely replaced every 3 hours.
Your baby is now about 10 inches (25 centimeters) long and weighs about 12 ounces (about 340 grams).
Your baby grows quickly in size and strength from now until birth. The skin is wrinkled and red and covered with a substance of oil and cells called vernix. The baby can open and close its eyes and can hear sounds. The baby's fingerprints and footprints have formed.
At the end of this month cells inside the baby's lungs begin to make a fatty substance called surfactant. This substance helps the baby breathe after birth. Because babies are still so small and the lungs are not fully developed at this stage, they usually cannot live outside the uterus without very specialized care.
By the end of the 6th month, your baby is about 12 inches (30 centimeters) long and weighs about 1 to 1 and 1/2 pounds (450 to 680 grams). The sixth month is the last month of the second trimester.
The seventh, eighth, and ninth months are the third and final trimester of pregnancy.
The baby exercises by kicking and stretching. The bones are hardening. The lanugo begins to disappear from the face, and the baby may have more hair on its head. The baby can make grasping motions with its hands. If the baby is a boy, the testicles begin to move down from the belly into the scrotum.
Your baby is nearly 15 inches (38 centimeters) long and weighs 2 to 3 pounds (900 to 1350 grams). The baby now has a better chance of survival if born early.
Your baby is getting too big to move around much, but its kicks are stronger, and you may be able to see the outline of a small heel or elbow against your abdomen. If your baby is a boy, his testicles have descended into his scrotum. Although the baby's bones keep hardening, the bones of the head are soft and flexible. The baby is getting antibodies from the mother that will help protect the baby against illness for the first few months after birth. Sometimes the baby will have hiccups.
Your baby is now 16 to 18 inches (40 to 46 centimeters) long and weighs 4 to 5 pounds (1800 to 2270 grams).
Your baby now becomes plumper and gains about 1/2 pound (225 grams) each week. The baby is getting ready for birth. The baby usually settles head first in the birth canal with its knees against its nose in what is called the fetal position. Sometimes the baby will settle with its buttocks in the birth canal. This is called the breech position.
In a first pregnancy, many women feel the baby drop lower in the pelvis during the last 2 to 3 weeks, right around the due date. In other pregnancies, women may not feel their baby drop until just before or even after labor has begun. The baby drops when its head starts to enter your pelvis. The bones of the head are soft and flexible for delivery through the birth canal. The lanugo (body hair) is almost all gone.
Your baby is now about 20 inches (50 centimeters) long and weighs 6 to 9 pounds (2700 to 4000 grams). No one knows what makes labor start. You may go into labor and give birth to the child any time between the 37th and 42nd weeks of pregnancy. A full-term pregnancy is from 39 to 41 weeks since the first day of the last menstrual period.