Fetal Development: Month by Month

Human fetus in utero at 8 weeks

Your Baby's Growth: Conception to Birth

Congratulations on becoming pregnant! We are sure you are curious about how your pregnancy will progress, and how your baby will develop week to week over the next few months. In this slideshow we will take a look inside the womb to see how a baby develops through the first, second, and third trimesters.

sperm fertilizing egg


Step one of conception is when the sperm penetrates the egg to complete the genetic make-up of a human fetus. At this moment (conception), the sex and genetic make-up of the fetus begins. About three days later, the fertilized egg cell divides rapidly and then passes through the Fallopian tube into the uterus, where it attaches to the uterine wall. The attachment site provides nourishment to the rapidly developing fetus and becomes the placenta.

fetal development at 4 weeks

Baby's Development at 4 Weeks

After 4 weeks, the basic structures of the fetus have begun to develop into separate areas that will form the head, chest, abdomen, and the organs that are contained within them. Small buds on the surface will become arms and legs. A home pregnancy test should be positive at this stage of development (most tests claim positive results one week after a missed period).

fetal development at 8 weeks

Baby's Development at 8 Weeks

At 8 weeks, the fetus is about one-half an inch long (1.1cm). Facial features such as developing ears, eyelids, and nose tip are present. The limb buds are now clearly arms and legs, while the fingers and toes are still developing.

Human fetus in utero at 12 weeks

Baby’s Development at 12 Weeks

At 12 weeks, the fetus has grown to about 2 inches (4.4cm) in length and may begin to move by itself. The fingers and toes are discernible and the fetal heartbeat may be audible by Doppler ultrasound. The developing sex organs may be identified by ultrasound techniques.

Human fetus in utero at 16 weeks

Baby's Development at 16 Weeks

At 16 weeks, the fetus is about 4 and one-half inches long and resembles an infant; the eyes blink, the heartbeat is easier to locate, facial features (nose, mouth, chin and ears) are distinct, and the fingers and toes are clearly developed; the skin on the fingers and toes even have distinct patterns (fingerprints!). Women should be able to feel the uterus at about 3 inches (6.6 cm) below the belly button; this is the beginning of the "baby bump" (abdominal swelling due to an expanding uterus) in some women.

Human fetus in utero at 20 weeks

Baby's Development at 20 Weeks

At twenty weeks, the developing baby is about 6 inches long (13.2 cm) and may weigh about 10 ounces. The baby may begin to make movements that the mother can feel at about 19 to 21 weeks; this baby movement is termed "quickening". The baby at this stage of development can move its facial muscles, yawn, and suck its thumb. The expanding uterus at 20 weeks is felt at the level of the belly button.

4D (3D in real time) and standard ultrasound images

It's Time for an Ultrasound

In the US, women that have prenatal care usually have an ultrasound done at 20 weeks to determine that the placenta is attached normally and that the baby is developing without any problems. The baby's movements can be seen with Doppler imaging, and usually the sex of the baby can be determined at this time, so if you want to be surprised about the sex of your baby at delivery, let your doctor know before the Doppler ultrasound is started!

Shown here is a 2D ultrasound (inset) contrasted with a 4D ultrasound, both at 20 weeks.

4D ultrasound of 24-week fetus

Baby's Development at 24 Weeks

At 24 weeks, the baby may weigh 1.4 pounds and can respond to sounds. Doppler studies show the sound response by measuring movement and heartbeat rates. Sometimes the baby will develop hiccups that the mother can feel! The baby's inner ear canals are developed at 24 weeks, so researchers speculate the baby can sense its position in the uterus.

4-D ultrasound of a fetus at 28 weeks

Baby's Development at 28 Weeks

At 28 weeks, the baby normally weighs about 2 and one-half pounds and has developed to the point that if the baby is birthed prematurely for any reason, the chances are good that the infant will survive, but usually would require a hospital stay. Your doctor may discuss signs of premature labor and suggest you (and your partner) take classes on what to do at the time of delivery of your full-term baby.

Pregnant woman being examined by doctor

Baby's Development at 32 Weeks

At 32 weeks, many babies weigh about 4 pounds, and have movements that the mother can feel. Your doctor may ask you to make notes about the baby's movements and discuss breastfeeding and other options along with scheduling visits every two weeks until you deliver the baby. Some women begin to leak a yellowish fluid from their breasts around this time; this is normal and the fluid is termed colostrum and indicates the breasts are primed to start producing milk for the newborn baby.

fetus in utero at term

Baby's Development at 36 Weeks

At 36 weeks the baby is about ready to be delivered and has reached an average length of 18.5 inches from head to heel length and weighs about 6 pounds. However, baby weight and length are quite variable and are influenced by the baby's parental genetics, the baby's sex, and many other factors. During this time, the baby has begun to rotate itself into the delivery position of head first into the pelvis. At 37 weeks, the baby has completed development of all organ systems to a level that should allow it to survive and continue its growth outside the uterus without any close hospital monitoring that is usually done with premature babies; consequently, the pregnancy is considered "at term" at 37 weeks and beyond.

Newborn baby at birth


Delivery, due or birth date is calculated by estimating a 40 weeks delivery date, calculated after the first day of the mother's last period. This is an estimated date; the normal vaginal delivery birth can occur easily between 38 and about 42 weeks and is considered an early or late term pregnancy. However, most babies are delivered before 42 weeks. Depending on various circumstances and complications, the doctor may need to induce labor and delivery in some women, while others may require a surgical delivery (Caesarean section or C-section). For most people, especially first-time parents, birth of an infant is a life-changing event!



  1. Copyright © LookatSciences / Phototake – All rights reserved.
  2. Dr. David M. Phillips / Visuals Unlimited / Getty Images
  3. 3D4Medical.com / Getty Images
  4. Copyright © Scott Camazine / Phototake -- All rights reserved.
  5. Copyright © LookatSciences / Phototake – All rights reserved.
  6. Nestle / Petit Format / Photo Researchers, Inc.
  7. © Lennart Nilsson Photography AB. All rights reserved worldwide.
  8. a) Dr.Benoit/Mona Lisa. Copyright © LookatSciences / Phototake -- All rights reserved. b)Vincenzo Lombardo / Photographer's Choice / Getty Images
  9. Dr. Benoit/Mona Lisa. Copyright © LookatSciences / Phototake -- All rights reserved.
  10. © Lennart Nilsson Photography AB. All rights reserved worldwide.
  11. Jose Manuel Gelpi Diaz / iStockphoto
  12. © Lennart Nilsson Photography AB. All rights reserved worldwide.
  13. © Yoav Levy / Phototake -- All rights reserved.


  • UpToDate: "Assessment of Fetal Lung Maturity"
  • American Pregnancy Association: "Fetal Development"
WebMD does not provide medical advice, diagnosis or treatment. See additional information