Changes by weeks of gestation
FETAL PERIOD
From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.
Weeks 10-12
Gestational age: 9-11 weeks old.
Embryonic age: Weeks nr 8-10. 7-9 weeks old.
Embryo measures 30 mm-8 cm (1.2-3.2 inches) in length.
Intestines rotate.
Facial features continue to develop.
The eyelids are more developed.
The external features of the ear begin to take their final shape.
The head comprises nearly half of the fetus' size.
The face is well formed
The eyelids close and will not reopen until about the 28th week.
Tooth buds, which will form the baby teeth, appear.
The limbs are long and thin.
The fetus can make a fist with its fingers.
Genitals appear well differentiated.
Red blood cells are produced in the liver.
Weeks 13 to 16
Gestational age: 12-15 weeks old.
Embryonic age: Weeks nr 11-14. 10-13 weeks old.
The fetus reaches a length of about 15 cm (6 inches).
A fine hair called lanugo develops on the head.
Fetal skin is almost transparent.
More muscle tissue and bones have developed, and the bones become harder.
The fetus makes active movements.
Sucking motions are made with the mouth.
Meconium is made in the intestinal tract.
The liver and pancreas produce fluid secretions.
Week 19
Gestational age: 18 weeks old.
Embryonic age: Week nr 17. 16 weeks old.
The fetus reaches a length of 20 cm (8 inches).
Lanugo covers the entire body.
Eyebrows and eyelashes appear.
Nails appear on fingers and toes.
The fetus is more active with increased muscle development.
"Quickening" usually occurs (the mother can feel the fetus moving).
The fetal heartbeat can be heard with a stethoscope.
Week 23
Gestational age: 22 weeks old.
Embryonic age: Week nr 21. 20 weeks old.
The fetus reaches a length of 28 cm (11.2 inches).
The fetus weighs about 725 g
Eyebrows and eyelashes are well formed.
All of the eye components are developed.
The fetus has a hand and startle reflex.
Footprints and fingerprint continue forming.
Alveoli (air sacs) are forming in lungs.
Week 27
Gestational age: 26 weeks old.
Embryonic age: Week nr 25. 24 weeks old.
The fetus reaches a length of 38 cm (15 inches).
The fetus weighs about 1.2 kg
The brain develops rapidly.
The nervous system develops enough to control some body functions.
The eyelids open and close.
The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth.
The respiratory system, while immature, has developed to the point where gas exchange is possible.
Week 31
Gestational age: 30 weeks old.
Embryonic age: Week nr 29. 28 weeks old.
The fetus reaches a length of about 38-43 cm (15-17 inches)
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The fetus weighs about 2 kg.
The amount of body fat rapidly increases.
Rhythmic breathing movements occur, but lungs are not fully mature.
Thalamic brain connections, which mediate sensory input, form.
Bones are fully developed, but are still soft and pliable.
The fetus begins storing iron, calcium, and phosphorus
Week 35
Gestational age: 34 weeks old.
Embryonic age: Week nr 33. 32 weeks old.
The fetus reaches a length of about 40-48 cm (16-19 inches).
The fetus weighs about 2.5 to 3 kg
Lanugo begins to disappear.
Body fat increases.
Fingernails reach the end of the fingertips.
a baby born at 36 weeks has a high chance of survival, but may require medical interventions.
Weeks 36 to 39
Gestational age: 35-38 weeks old.
Embryonic age: Weeks nr 34-37. 33-36 weeks old.
The fetus is considered full-term at the end of the 37th week of gestational age.
It may be 48 to 53 cm (19 to 21 inches) in length.
The lanugo is gone except on the upper arms and shoulders.
Fingernails extend beyond fingertips.
Small breast buds are present on both sexes.
Head hair is now coarse and thickest.
