Fetal Development: Week by Week
- Conception occurs when the male and female gametes unite.
- The embryo implants in the endometrium.
- A neural tube forms; it will ultimately become the brain and spinal cord.
- The circulatory system rapidly forms (including the umbilical cord), and the heart begins to beat by day 21.
- Average positive pregnancy test usually occurs around day 24.
- Hands and wrist joints are forming.
- All cranial nerves are now present, and limb nerves are forming.
- Subtle movement begins.
- Brainwave activity has begun.
- Arm and leg bones form, along with tooth buds.
- Eye lids are present and growing and a primitive nasal cavity forms.
- Vital organs are continuing to form, including the kidneys, liver, pancreas, lungs, stomach, and intestines.
- The head rotates and legs move; the baby is also able to hiccup.
- Hands can touch each other and fingers can overlap.
- The heart, which began beating weeks ago, is nearly complete and is beating very quickly now (peaking between 165 and 170 beats per minute).
- Feet begin to have tiny notches that will become toes, and knee joints form.
- Hair appears around eyebrows and mouth.
- Urine production and release begins.
- Joints are similar to those of an adult.
- Fingers and toes are webbed and short but continuing to grow.
- Eyelids are completely fused together.
- The head is now much larger than the rest of the body.
- The baby can suck his/her thumb and move the tongue.
- Early vocal cords are present and the baby begins to yawn.
- Fingernails and toenails start to grow, and unique fingerprints are now present.
- Bones are beginning to solidify.
- Eyes now roll down reflexively.
- Buds form that will eventually be secondary teeth.
- Nose and lips are completely formed; complex facial expressions begin.
- Reproductive organs are well-formed and gender can now be seen on ultrasound.
- Intestines absorb water and glucose.
- Weight increases 60% this week.
- Hands touch mouth up to 50 times an hour.
- Bile production begins in liver.
- Bowel movements begin.
- There are taste buds all over mouth.
- Teeth, as well as other tissues, are growing.
- Cilia are lining airways.
- Most of the body is sensitive to touch.
- Permanent kidneys appear.
- Girls move their jaws more than boys do.
- Cerebellum resembles that of an adult, and cerebral cortex now has four lobes.
- Fat deposits present in cheeks.
- Touch to mouth causes the baby to turn away.
- Stem cells arrive in bone marrow.
- Body fat emerges throughout body.
- Glucagon is present in the bloodstream.
- The mother can begin to feel the baby’s movements.
- Tooth enamel is forming.
- Bronchial tree is nearly complete.
- The baby has hormonal stress response to invasive procedures.
- The retina now has discrete layers.
- The cerebral cortex is now active.
- Meconium (made up of products of cell loss, digestive secretion, and swallowed amniotic fluid) is accumulating in the bowel.
- Eyes have now shifted to their final position, and ears are almost done moving.
- A cream-like substance, called vernix caseosa, protects the skin from exposure to amniotic fluid.
- The bones in the inner ear and nerve endings in the brain have developed enough that sounds can be detected.
- Sweat glands have formed.
- Insulin secretion begins.
- Melanin production begins.
- In female babies, the number of oogonia, which will become eggs, reaches its height; oogonia production ceases permanently.
- In male babies, the prostate gland begins to develop.
- Daily cycles in biological rhythms, such as sleep, begin.
- All skin layers and structures are now present.
- The cochlea in the inner ear reaches adult size and the baby can now respond to sounds.
- The eyelids separate and the eyes now open and close.
- The head is now about 20 centimeters in circumference.
- This is the earliest that low-levels of surfactant, a substance necessary for gas exchange in the lungs, is produced.
- Legs are reaching their correct proportions.
- The brain is growing very quickly; nerves begin making complex connections and will continue to do so until about 5 years after birth.
- Hair begins to grow on the scalp.
- A baby born prematurely at this stage has more than a 50% chance of survival.
- Cornea structure is now developed.
- White blood cells are developing to allow the baby to fight off infection.
- As the baby gains weight, the skin becomes less transparent and wrinkly.
- Hearing becomes clearer as inner ear bones harden.
- The brain weighs about 100 grams this week, and is about to start a major growth spurt, during which its size will increase 400-500%.
- The baby’s nostrils open.
- Daily movement helps work out developing muscles.
- The baby can hear things outside the womb now, and will have a blink-startle response to a loud, sudden noise.
- Loud noises also cause an increase in heart rate and movement.
- Surfactant is now definitely present, so gas exchange outside the womb is possible.
- Rods and cones, which allow the eye to detect light and color, are now present.
- The baby now has the ability to taste, which affects the rate at which he or she swallows amniotic fluid. If the mother eats sweet food, he will swallow faster, while bitter food leads to slower swallowing.
- The intestinal lining now contains all adult cell types.
- The sense of smell is now present.
- Tear production may begin.
- Additional fat deposits continue to decrease wrinkles.
- Eyelashes are now growing.
- Pupils react to light.
- Fat deposits continue to build up so that the baby can maintain body temperature after birth.
- The placenta is maintaining a higher temperature than that of the rest of the mother’s body, roughly 100.7 to 102.5 degrees Fahrenheit.
- The baby can now differentiate between high and low frequency sounds from outside the womb.
- Bone marrow takes over the production of red blood cells from the liver.
- Smooth brain tissue is developing wrinkles and folds as brain mass increases.
- By now, the baby can do somersaults by a series of movements similar to walking.
- The baby’s head is no longer disproportionate to the rest of the body.
- Eyes can move around in their sockets, and the rest of the senses are functioning.
- Movement becomes less pronounced as space in the uterus becomes more cramped.
- “Practice” breathing motions continue to be common, although there is no air in the womb.
- The cerebral cortex now consists of six layers.
- The head is now about 30 cm in circumference.
- The baby is most likely having dreams by this point.
- Growth will begin to slow down now, but weight at birth will still be approximately twice what it is now.
- A doctor will now begin noting the position of the baby in hopes that he or she will be born headfirst.
- The baby’s memory begins working, as he or she will typically prefer songs heard in the later part of pregnancy to ones heard only after birth.
- The formation of glomeruli, blood-filtering capillaries in the kidneys, is complete.
- Muscles of the lower esophagus are now functional.
- The baby’s skull bones have not fused and are very pliable.
- Amniotic fluid is at its highest level and will stay at this level until birth.
- The baby may settle into the head down position at this point, though it may happen later in pregnancy.
- The mother may gain additional weight at this point as the baby is doing the same.
- Rapid weight gain includes increase in brain weight as the myelin sheath around nerves is thickening during the last weeks of pregnancy.
- The immune system is continuing to mature so that the baby can fight infections.
- Pressure from the baby being head down may cause the mother to urinate more frequently.
- The baby has now developed a firm grip.
- He or she will now gain ½ to ¾ of a pound each week.
- Lung development is almost complete and lungs are almost ready for birth.
- Surfactant production accelerates.
- Brain weight is approximately 300 grams.
- The immune system continues to develop as the baby receives antibodies from his or her mother.
- Fetus drinks an estimated 15 ounce of amniotic fluid each day.
- The baby is becoming more coordinated and can now grasp things.
- The pregnancy is now considered full-term and a doctor will not try to stop labor if it starts.
- Breathing motions will sometimes cause the baby to get hiccups.
- Vernix, the waxy substance on the skin, is mostly gone now; only a small amount remains to help the baby travel through the birth canal smoothly.
- The baby is gaining a half ounce to an ounce each day.
- The mother’s appetite may decrease as there is less room for everything in her body, including her stomach.
- The baby now has over 300 bones as the skeleton continues to develop.
- All organs are now ready for birth, including the lungs.
- The umbilical cord is now about 20 inches long and a half inch thick; due to crowding, it is not uncommon for it to get knotted.
- Baby is born!
- After birth, most of the vernix will be gone, though some traces may still be noticeable.
- The head may be cone-shaped after birth due to the passage through the birth canal, but will become round again in the next few days.
- The baby will be capable of crying at birth, though tears may not appear because of underdeveloped tear ducts.
Websites on Fetal Development
- Just the Facts is an interactive website that shows visitors how babies develop in the womb through pictures and videos. The site also has teaching resources available.
- The Endowment for Human Development has detailed web and DVD resources that explain the development of the unborn in a scientific, yet accessible way.
Books on Fetal Development
- A Child is Born – Lennart Nilsson
- Life – Lennart Nilsson
- The Developing Human: Clinically Oriented Embryology
Videos on Fetal Development
- In the Womb – National Geographic
- In the Womb Multiples – National Geographic
- From Conception to Birth – Discovery Channel