Your baby begins life as a tiny cluster of cells, but during your pregnancy, he becomes a unique baby with facial features, a beating heart, and kicking legs. Nine months might seem like a long time to be pregnant, but growing a person takes time! Learn all about your baby’s fetal development below.
Once you understand what is going on “in there” during pregnancy, you will see why. Find out what’s happening with your baby’s development each month.
Fetal Development – First Trimester
The first trimester lasts from conception to 12 weeks, which is generally the first three months of pregnancy. During this trimester, your fertilized egg will change from a small grouping of cells to a fetus that is beginning to develop your baby’s features.
Month 1 (weeks 1 through 4)
A water-tight sac will form around the fertilized egg, gradually filling with fluid. This is the amniotic sac, and it helps protect your growing baby. The placenta, a round, flat organ that transfers nutrients from you to your baby, also forms during this time. Your baby’s blood cells start taking shape, and circulation will begin.
In these first four weeks, the start of your baby’s face will begin to take form. Your baby’s mouth, throat, and lower jaw are developing during this time too. By the end of the first month, your baby will be about 1/4 inch long, and their tiny “heart” tube will beat 65 times a minute.
Month 2 (weeks 5 through 8)
Your baby’s face is continuing to develop, with his ears beginning to form as little folds of skin at the side of his head. Tiny buds are forming that will soon grow into arms and legs. Fingers, toes, and eyes are also forming.
Your baby’s brain, spinal cord, and central nervous system are well-formed during month two of pregnancy. Her sensory and digestive organs start to develop too, and bone starts to replace cartilage. At about six weeks, your doctor will be able to hear your baby’s heartbeat. By the end of week eight, your baby is about one inch long.
Month 3 (weeks 9 through 12)
Your baby’s fingers, hands, arms, feet, and toes are fully formed, and she will start to explore a bit by opening and closing her mouth and fists. Her ears are formed, and fingernails and toenails are beginning to develop. Teeth start to form under her gums.
Your baby is fully formed by the end of the third month; all of his organs and limbs are there and will continue to grow. His circulatory and urinary systems are also working, and his liver produces bile.
At the end of week 12, your baby is about four inches long and weighs about 1 ounce. Your chances of having a miscarriage will drop after three months since most of your baby’s fetal development has taken place by now.
Second Trimester
Many women say the second trimester is the best one because morning sickness has probably faded by now. Your baby will start to develop facial features, and you might start to feel movement as she flips and turns in your uterus.
During the second trimester, many women find out whether their baby will be a boy or girl. This is typically done by ultrasound at around 20 weeks.
Month 4 (weeks 13 through 16)
In month four, your baby’s fingers, toes, eyelids, eyebrows, eyelashes, nails, and hair are formed. She can even suck her thumb, yawn, make faces, and stretch. Her nervous system is starting to work. By the end of week 16, your baby is about six inches long.
Month 5 (weeks 17 through 20)
Many women start to feel their baby moving around at this stage. That’s because he’s developing muscles and exercising them. This first movement feels like a flutter and is called quickening.
Your baby’s hair will start to grow on her head. Her back, shoulders, and temples are covered by something called “lanugo,” a soft fine hair that protects her. Her skin is covered with a whitish coating called vernix caseosa to protect her skin from being exposed to the amniotic fluid.
By the end of month five, your baby is about 10 inches long and weighs from 1/2 to 1 pound.
Month 6 (weeks 21 through 24)
During month six, your baby’s skin is wrinkled and reddish, and his veins are visible through translucent skin. His eyelids start to part, and his eyes open.
Your baby responds to sounds by moving, and you might feel jerking motions if your baby hiccups. By the end of week 24, your baby is about 12 inches long and weighs around two pounds.
Month 7 (weeks 25 through 28)
Your baby’s hearing is fully developed, and he’s forming body fat. He’ll start moving more and respond to sound and light. The amniotic fluid begins to decrease. Your baby is likely to survive after the seventh month if born prematurely. At the end of week 28, your baby weighs from two to four pounds and is about 14 inches long.
Fetal Development – Third Trimester
During the final stretch of pregnancy, many women are tempted to start the countdown to their due date and hope labor comes early. But this final stage of development is important because they help your baby for birth. Throughout the third trimester, babies gain weight quickly, adding body fat that will help them after birth.
Even though popular culture only talks about nine months of pregnancy, you may actually be pregnant for ten months. A full-term pregnancy is 40 weeks, which may take some women into the tenth month. Some women even go past their due date by a week or two.
Your doctor will monitor you closely as you get nearer to your due date, so that means more frequent appointments. If your due date has passed and you don’t go into labor, your doctor may use medications to make you go into labor and have the baby. Make sure to talk to your Adoption Coordinator and doctor during your trimester about your hospital and birth plan.
Month 8 (weeks 29 through 32)
Your baby keeps maturing and forming reserves of body fat. You may feel more kicking. Your baby’s brain is developing rapidly, and she can see and hear. Most of her internal systems are well developed. Your baby weighs as much as five pounds and is about 18 inches long.
Month 9 (weeks 33 through 36)
During this stage, your baby continues to grow and mature and your baby’s fetal development is almost complete. His lungs are almost fully developed at this point. Your baby can blink, turn his head, close his eyes, grasp firmly, and respond to sounds, light, and touch. He’s about 17 to 19 inches long and weighs from 5 ½ pounds to 6 ½ pounds.
Month 10 (Weeks 37 through 40)
You could go into labor any time during this final month. Many women notice less movement because space is tight. Your baby’s position should change to get ready for birth and drop down into your pelvis.
Your baby is now ready to meet the world! They weigh about seven pounds and are 18 to 20 inches long.
Editor’s Note: This article was originally published on September 25, 2008, and has since been updated.
Mardie Caldwell, C.O.A.P., is nationally recognized as an expert on open adoption. A Certified Open Adoption Practitioner (C.O.A.P.), Caldwell is the founder of Lifetime Adoption Center, established in 1986. She has assisted in over 2,000 successful adoptions and was one of the first adoption professionals on the Internet.
Caldwell's life work is dedicated to educating and helping birth parents find the right adoptive parents for their child. She spreads the word about modern adoption through speaking appearances, webinars, online resources, and as a podcast show host.
She has written several award-winning books, including So I Was Thinking About Adoption, the first book of its kind. There are many reasons women choose adoption, and this short book is a comprehensive resource to make the best plan for you and your baby. Caldwell wrote So I Was Thinking About Adoption as a handy guide to the details of the adoption process.
Caldwell has made over 150 media appearances, including ABC News, CBS News, Larry King Live, CNN Headline News, NBC's The Today Show, CNN's The Campbell Brown Show, NBC News, KGO Newstalk Radio, CNN's Black in America II, MSNBC, Fox, PBS, BBC, and Dr. Laura.
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