A normal pregnancy lasts about 40 weeks. You start counting these weeks of pregnancy from the first day of your last menstrual period.
The first three months of pregnancy are called the first trimester, the next three months are called the second trimester, and the last 3 months are called the third trimester.
A baby grows and changes a lot during this time. It can be helpful to know how your baby changes from month to month.
About 5 to 7 days after your baby is conceived (when the sperm fertilizes the egg), the fertilized egg attaches to the lining of the uterus. This is called implantation. The egg then starts to grow in the uterus. At this time the baby is called an embryo.
Shortly after the egg implants in the uterus, the placenta and umbilical cord start to form. The placenta and umbilical cord bring food and oxygen to your baby and carry away the baby’s wastes.
In another week the baby has a spinal cord and starts developing the bones of the spine. The heart also forms, and it starts to beat at 5 to 6 weeks. By the end of the first 6 weeks of pregnancy, your baby’s head and lower body are formed. The eyes, brain, mouth, inner ears, and digestive system are starting to form, and your baby has tiny buds that will become the arms and legs. Your baby is in a sac of fluid called the amniotic sac. This sac protects the baby from bumps and pressure.
By the end of 6 weeks your baby is about a half inch long (1 centimeter) and weighs less than an ounce (a few grams).
The second month is especially important in the development of your baby. Exposure to drugs, viruses, or poisonous chemicals (like pesticides) could affect the babyâ€™s development at this time and cause birth defects.
Your baby’s development is very fast during this time. By the end of the second month, all of your baby’s major body organs, 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 babyâ€™s head is about the size of the rest of its body.
During the third month the babyâ€™s fingers and toes have soft nails, and tooth buds have formed under the gums. Hair may start to appear on the head. There is a bit of a nose and the lips are forming. The kidneys develop and start draining urine into the bladder.
The baby may open and close its mouth and start moving its hands, legs, and head. It is still too early, however, to feel this movement.
By the end of the third month, your baby is completely formed. Your baby is 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.
By the fourth month, eyebrows and eyelashes start to appear. The rest of 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.
The baby moves, kicks, sleeps, and wakes up. You may start to have a slight feeling of the babyâ€™s movement in your lower belly. It may feel like bubbles or fluttering.
At the end of the fourth month, your baby is 6 to 7 inches (16 to 18 centimeters) long and weighs about 6 ounces (170 grams).
During the fifth month, the internal organs are more fully formed, and the fingernails have grown to the tips of the fingers. Your baby has a lot more brain cells. Solid waste, called meconium, starts to form in the bowels. The baby may suck its thumb. The baby is much more active, turning from side to side and head over heels. The baby swallows fluid and urinates into the amniotic sac.
If you have an ultrasound, which uses sound waves to show pictures of the baby, you may be able to learn the sex of the baby.
Your baby is now about 10 inches (25 centimeters) long and weighs about 12 ounces (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 start 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 womb without very special 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.
During the seventh month, the baby exercises by kicking and stretching. The bones are getting harder. The lanugo starts to go away from the face, and the baby may have more hair on its head. The baby can make grasping motions with its hands.
At the end of the seventh month, 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. You may be able to see the outline of a small heel or elbow pressing against your belly. 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 gets plumper and gains about 1/2 pound (225 grams) each week. The baby is getting ready for birth. Organs like the heart, lungs, and kidneys will be able to work on their own after birth. The lanugo is almost all gone. The baby usually settles into a position with its head down in the birth canal and knees against its nose in what is called the fetal position. Sometimes the baby will settle with its buttocks in the birth canal into a position called the breech position.
You may feel the baby drop lower in the pelvis during the last 2 to 3 weeks, right around the due date. Or you may not feel your baby drop until just before or even after labor has begun. Although most of the baby’s bones have hardened, the bones of the head stay soft and flexible for delivery through the birth canal.
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 any time between the 37th and 42nd weeks of pregnancy.
Developed by RelayHealth.
Adult Advisor 2015.1 published by RelayHealth. Last modified: 2012-11-28 Last reviewed: 2013-12-22
This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.
Normal Growth of a Baby During Pregnancy: References
Cunningham, F., K. Leveno, S. Bloom, J. Hauth, L. Gilstrap, K. Wenstrom. Williams Obstetrics. 22nd ed. The Mcgraw Hill Companies, Inc. 2008. Accessed September 23, 2009 from http://www.accessmedicine.com.
Gibbs, R. B. Karlan, A. Haney, I. Nygaard. Danforthâ€™s Obstetrics and Gynecology. 9th ed. Lippincott Williams and Wilkins, 2008. Accessed on December 29, 2011, from http://www.ovidsp.tx.ovid.com.
Riley, LE, and A.R. Stark. Guidelines for Perinatal Care. 7th ed. AAP and ACOG. 2012.