Baby Development Timeline

The growth and development of a baby is an incredible experience. From the beginning, we can see God’s beautiful creation of a human.







Fertilization: the joining of the egg and the sperm to form a unique human being.  The baby already contains a full set of DNA from the mother and father, which determines sex, eye color, hair color and other traits.

Week 4


The embryo is about the size of a poppy seed. When the embryo reaches the uterus, it will burrow into the uterine wall for nourishment. The placenta, which will nourish the baby throughout the pregnancy, also begins to form.  Hormones produced by the embryo have stopped the mother’s menstrual cycle.  Over the next six weeks, all of your baby’s organs will begin to develop, and some will start to function.

Weeks 5 – 6 

The baby’s tiny heart begins to beat – at twice the rate of yours. His brain, spinal cord, and other organs also begin to form.  His entire form is only about the size of a sesame seed. His heart is pumping blood.  Basic facial features (like eyes and nostrils) are beginning to form. His body begins to take on a c-shaped curvature. Small buds will soon become arms and legs.  By the end of this week, he may be 1/6 to 1/4 inch long.

Week 7


The baby’s brain and face are rapidly developing. The arm buds that developed last week now take on the shape of paddles. She has developed her own blood type, unique from her mother’s.

Week 8


The baby’s arms and legs are growing longer, and her now has little fingers as well as a nose and upper lip. He may begin to move this week, but the mother won’t be able to feel it yet. The trunk of the baby’s body is beginning to straighten and he is about 5/8 of an inch long.

Weeks 9 – 11 

The baby’s eyes have developed, though eyelids are fused shut for now. Vital organs – such as kidneys, intestines, brain, and liver – are starting to function. Tiny fingernails and toenails are forming. Bones are beginning to harden, and genitalia are developing externally. She can hiccup, though it’s too soon to feel it.

Week 12


The baby’s heartbeat can be heard at a prenatal checkup, (it may have already been heard at an early ultrasound). He is just over 2 inches long and weighs about half an ounce.

2nd Trimester

Weeks 13-16


The baby can make sucking motions with her mouth now and her nervous system allows her to feel pain. Muscle tissue and bone continue to form, creating a more complete skeleton. He can make active movements including kicking and even somersaults. More developed facial muscles may lead to various facial expressions such as squinting or frowning. He can see light that filters in from outside the womb, even though eyelids are still shut.

Weeks 17-18


If the mother hasn’t felt her baby move yet, she probably will in the next few weeks. It will take a couple of weeks longer for other people to feel the baby’s movements from the outside.  Fat begins to develop under the baby’s skin. The fat will provide energy and help keep her warm after birth. Her ears begin to stand out on the sides of her head. As the nerve endings from her brain “hook up” to the ears, she  may hear the mother’s heart beating, her stomach rumbling or blood moving through the umbilical cord. She may even be startled by loud noises.

Week 20


Fingernails and toenails appear and the baby’s sex organs are visible on ultrasound.  By now the baby may be about 6 1/3 inches long and weigh more than 11 ounces.

Weeks 21- 24


The baby’s sense of movement has developed, and the sense of hearing continues to improve. He may sometimes be seen squirming under the surface of your belly. His brain is growing very quickly.  He is becoming more active, can swallow and is regularly sleeping and waking. The tongue will soon develop taste buds. Fingerprints and footprints are forming. Hair is beginning to grow on his head.  With medical care, some babies born these weeks may be able to survive. He is almost 12 inches long and weighs just over a pound.

Weeks 25-27

The baby’s hands are fully developed. She can open and close her eyes.  Her lungs, liver and immune system are continuing to mature.  She has been doing “practice breathing” by inhaling and exhaling amniotic fluid, and if born now her lungs could function (with medical help).

3rd Trimester

Weeks 28 – 30


The baby is gaining weight.  The bones are fully developed, but they’re soft and pliable.  He has eyelashes and eyesight is improving.  He weighs about 2¼ lbs and is about 15 inches long.

Weeks 31-34


The baby’s body begins absorbing vital minerals, such as iron and calcium and has grown little fingernails and toenails.  She’s almost 17 inches long and weighs 3¾ pounds.  Thirty-three weeks into pregnancy, her pupils can constrict, dilate and detect light entering her eyes.

Week 37

The baby is now considered full-term. His lungs should work fine if he’s born now, but ideally he will stay in the womb a bit longer.

Week 40 – Birth

The baby is due and ready for life outside the womb. The average weight of a newborn is about 7 1/2 pounds, and the average length is about 20 inches.


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