Fetal Development: Week-by-Week Guide to Your Baby’s Growth

Fetal development is the process by which a fertilized egg grows into a newborn over about 40 weeks of pregnancy. It happens in three trimesters. During your first (weeks 1-13), your baby’s major organs form. Then, in the second (weeks 14-27), your baby grows quickly, and you may start to feel movement. In the third (weeks 28 to birth), your baby gains weight and their lungs mature.

Nine months might seem like a long time to be pregnant, but growing a person takes time! Below you’ll find what’s happening with your baby’s fetal development each week, the prenatal checkups and tests to expect in each trimester, and what each stage can mean for your adoption plan.

A note on counting weeks: Doctors count pregnancy from the first day of your last period (gestational age), not from conception. That’s why you’re considered “two weeks pregnant” on the day you conceive, and why a full pregnancy is about 40 weeks, or closer to ten months than nine.

Fetal Development at a Glance

Fetal Development: First Trimester (Weeks 1-13)

During the first trimester, a fertilized egg becomes an embryo with a beating heart and the beginnings of every major organ. Morning sickness and fatigue are common because your body is working hard.

Weeks 1-2: Getting ready for conception

Your period ends, and your body prepares to ovulate, usually around days 11-19 of your cycle. If a sperm fertilizes the egg, they form a single cell with 46 chromosomes called a zygote, the starting point for a new human being.

Week 3: The journey to the uterus

The fertilized egg spends a few days traveling down the fallopian tube, dividing into more and more cells. At this stage it’s called a morula, and then a blastocyst.

Week 4: Implantation

About 6 to 10 days after conception, the blastocyst implants in the lining of your uterus. The amniotic sac, a fluid-filled sac that protects your baby, begins to form, along with the placenta, which passes nutrients from you to your baby. The placenta also starts producing hCG, the hormone a pregnancy test detects.

Weeks 5-6: A heartbeat begins

The brain, spinal cord, and heart start to develop, and your baby’s heart begins beating in a steady rhythm. Tiny buds that will become arms and legs appear, and the early structures of the eyes and ears form. The lungs, jaw, nose, and palate start forming, too. By week six, your baby is about ¼ inch long, and you can often see a heartbeat on a vaginal ultrasound.

Weeks 7-8: Every essential organ begins to form

By week seven, every essential organ has begun to form, even though your baby still weighs less than an aspirin. Eyelids, the tongue, and hair follicles start to appear. Webbed paddles become fingers and toes, the ears continue developing, bones begin replacing cartilage, and muscles can contract. By the end of week eight, your baby is about ½ to 1 inch long, roughly the size of a bean.

Weeks 9-10: Almost a fetus

Your baby’s face is becoming more defined, the tail-like structure at the base of the spine disappears, and tooth buds appear. Your baby can open and close their fists. This is the last stretch of the embryonic period.

Week 11: Officially a fetus

After about week 10, your baby is called a fetus. The head makes up nearly half of their length, and the genitals begin to form, though it’s still too early to see on ultrasound. Your baby is about two inches long.

Weeks 12-13: Organs at work

Your baby’s eyelids close and stay closed until about week 27. The kidneys produce urine, the liver makes bile, and fingernails begin to form. By the end of week 13, your baby is about three inches long and weighs about 1 ounce. Your chance of miscarriage drops significantly after this point, since most of your baby’s early development has taken place.

Prenatal checkups and tests: first trimester

Your first prenatal visit usually happens around weeks 8-10, with checkups roughly once a month. Common tests and screenings include:

• Blood type and Rh factor, and an antibody screen
• A complete blood count, which checks for anemia and measures white blood cells and platelets
• Immunity to rubella (German measles)
• Screening for hepatitis B and C, HIV, syphilis, gonorrhea, and chlamydia
• Urinalysis, which checks for protein, sugar, and ketones
• A Pap smear, if you’re due for one
• An early ultrasound to confirm your due date and heartbeat
• Optional genetic screening, such as a blood test or nuchal translucency scan around weeks 10-14

What this means for your adoption plan: first trimester
Whether you’ve decided or are still exploring your options, you can request confidential information at any time with no obligation. Start prenatal care early, and talk with an adoption coordinator about what a plan could look like, including open adoption.

Fetal Development: Second Trimester (Weeks 14-27)

Many women say the second trimester is the best one because morning sickness has often faded. Your baby is growing quickly, and you’ll probably start to feel movement.

Weeks 14-16: Fingerprints and first movements

Your baby’s skin is nearly transparent, and a fine, soft hair called lanugo begins to grow on the head. Your baby starts sucking and swallowing small amounts of amniotic fluid. Fingerprints, the unique ridges that identify every person, form on the tiny fingers. Meconium, a dark, sticky substance, begins to collect in the intestines and will become your baby’s first bowel movement after birth. Sweat glands develop, and you may feel faint flutters. By week 16, your baby is about 4½ inches from head to rump (roughly 6 inches head to toe) and weighs about 4 ounces.

Weeks 17-20: Quickening

Many women first feel their baby move during this stretch, a flutter called “quickening.” Eyebrows and eyelashes grow in, and tiny nails form on the fingers and toes. Around week 20, the skin begins producing vernix caseosa, a white coating that protects it from the amniotic fluid. Your provider may be able to hear the heartbeat with a stethoscope. By week 20, your baby is about 10 inches long and weighs around 10 ounces.

Weeks 21-23: Looking more like a newborn

Lanugo now covers your baby’s body, and fat begins to develop under the skin, which becomes less transparent. All the parts of the eyes are in place, and the liver and pancreas are working hard to mature. Your baby is about 10-11 inches long and weighs 1 to 1¼ pounds.

Weeks 24-26: Sleep cycles, startle reflex, and viability

Your baby now has sleep and wake cycles, and you may start to notice a pattern in their movements. They also have a startle reflex and may jump at loud noises. Air sacs begin forming in the lungs, and the brain develops rapidly.

Around week 24, a baby born early may survive with intensive medical care, though every additional week in the womb improves the odds. By week 26, your baby is about 14 inches long and weighs nearly two pounds.

Week 27: Eyes begin to open

Your baby’s eyelids, closed since the end of the first trimester, may begin to open. At the end of this week, your baby is about 14½ inches long and weighs about 2 pounds.

Prenatal checkups and tests: second trimester

You’ll typically see your provider about once a month. Common tests include:

• Anatomy ultrasound (weeks 18-22): checks your baby’s growth and organs, and you may be able to find out your baby’s gender if you choose
• Maternal serum screening (weeks 15-22): a blood test that screens for conditions such as Down syndrome and neural tube defects
• Glucose screening (weeks 24-28): checks you for gestational diabetes

What this means for your adoption plan: second trimester
This is often a good time to review adoptive family profiles online, think about the kind of relationship and contact you’d like after birth, and ask your coordinator about pregnancy-related support available in your state. Counseling is also available if you’d like someone to talk to as you make your plan.

Fetal Development: Third Trimester (Weeks 28-40)

In the final stretch, your baby gains weight quickly and finishes the development needed for life outside the womb. Many women are tempted to start the countdown to their due date, but these last weeks matter.

Weeks 28-31: Storing fat and minerals

Your baby’s eyes open and can blink, and the brain is developing rapidly. Their bones are fully formed but still soft and pliable. Your baby stores calcium, iron, and phosphorus and builds up body fat. The lungs aren’t fully mature yet, but your baby is practicing rhythmic breathing movements. By week 31, your baby is about 16 inches long and weighs around three pounds.

Weeks 32-35: Gaining weight fast

Your baby gains weight more rapidly now, and the lanugo begins to disappear as the skin becomes smoother and less red and wrinkled. Around weeks 33-36, most babies settle into a head-down position to prepare for birth. By week 35, your baby is about 18 inches long and weighs 5 to 5½ pounds.

Week 36: Nearly there

Your baby’s lungs are almost fully developed, and they can blink, turn their head, grasp firmly, and respond to sound, light, and touch. Your baby is about 18-19 inches long and weighs around six pounds.

Weeks 37-40: Ready to meet the world

Weeks 37-38 are considered early term, and 39 weeks is considered full term. Your baby’s fingernails have grown long, and the lungs keep maturing right up until delivery. You’re also passing along antibodies that help protect your baby from illness after birth.

With less room to move, your baby’s movements may feel different, but they should still move regularly. Call your provider if you notice a significant decrease in movement. Most newborns are 19-21 inches long and weigh roughly six to nine pounds.

Prenatal checkups and tests: third trimester

Visits typically become every two weeks from week 28 and weekly from about week 36. Common tests include:

• A repeat antibody screen and blood count around week 28, plus an Rh immune globulin shot if you’re Rh-negative
• The glucose test, if you haven’t had it yet
• A Group B strep swab around weeks 36-37
• A growth ultrasound, if your doctor recommends one
• Cervical checks near your due date, and extra monitoring if you go past it

If your due date passes, your provider may talk with you about options for starting labor.

What this means for your adoption plan: third trimester
Now is the time to finalize your hospital and birth plan with your adoption coordinator and doctor, including who you’d like with you, how much time you’d like with your baby, and the support you’d like after delivery. Pack your hospital bag, and use guides on timing contractions and early labor signs so you know when to head to the hospital.

Frequently Asked Questions About Fetal Development

What are the stages of fetal development?

Fetal development runs about 40 weeks in three trimesters: the first trimester (weeks 1-13), when major organs form; the second (weeks 14-27), when your baby grows and starts moving; and the third (weeks 28 to birth), when your baby gains weight and the lungs mature.

What’s the difference between gestational age and fetal age?

Gestational age starts on the first day of your last menstrual period, so it runs about two weeks ahead of fetal age, which starts at conception. Doctors use gestational age.

What’s the difference between an embryo and a fetus?

An embryo is the early stage of development, through about week 10 of pregnancy. After that, until birth, your baby is called a fetus.

When does a baby’s heart start beating?

The heart begins beating around week five or six of pregnancy, and a heartbeat can usually be detected on ultrasound around week six.

When can I feel my baby move?

Most women feel the first flutters, called quickening, between about 16 and 25 weeks.

When do fingerprints form?

Fingerprints form during the second trimester, around weeks 14-16.

When is a baby viable?

Babies born around 24 weeks may survive with intensive medical care, and the chances improve with each additional week.

Wondering What Comes Next?

Understanding fetal development can make the weeks ahead feel less uncertain, and you don’t have to figure out your options alone. If you’re thinking about adoption, we’re here to answer your questions and help you build a plan that’s right for you and your baby.

Get Info Now

 

Sources

• Mayo Clinic: “Fetal development: The first trimester”
• Mayo Clinic, “Fetal development: The 2nd trimester”
• Mayo Clinic: “Fetal development: The 3rd trimester”
• The American College of Obstetricians & Gynecologists, “How Your Fetus Grows During Pregnancy”
• The American College of Obstetricians & Gynecologists, “Routine Tests During Pregnancy”
• The American College of Obstetricians & Gynecologists, “Prenatal Genetic Screening Tests”
• The American College of Obstetricians & Gynocologists, “Prevention of Group B Streptococcal Early-Onset Disease in Newborns”
• The Society of Maternal-Fetal Medicine, “ACOG Committee Opinion #579: Definition of term pregnancy”
• The American College of Obstetricians & Gynocologists, “Periviable Birth”
• March of Dimes, “What is full-term?”
• Cleveland Clinic, “Kick Counts”
• Cleveland Clinic, “Fetal Development”

Editor’s Note: This article was originally published on September 25, 2008, and has since been updated.