Fetal Development
Preembryonic Period
First 2 weeks after conception
Embryonic Period
Beginning day 15 through approximately week 8 after conception
Fetal Period
Week 9 after conception to birth
Week 1
Blastocyst is free-floating.
Weeks 2 to 3
Embryo is 1.5 to 2 mm in length.
Lung buds appear. Blood circulation begins.
Heart is tubular and begins to beat.
Neural plate becomes brain and spinal cord.
Week 5
Embryo is 0.4 to 0.5 cm in length.
Embryo is 0.4 g. Double heart chambers are visible.
Heart is beating. Limb buds form.
Week 8
Embryo is 3 cm in length.
Embryo is 2 g. Eyelids begin to fuse.
Circulatory system through umbilical cord is well established.
Every organ system is present.
Week 12
Fetus is 6 to 9 cm in length. Fetus is 19 g.
Face is well formed. Limbs are long and slender.
Kidneys begin to form urine. Spontaneous movements occur.
Heartbeat is detected by Doppler transducer between 10 and 12 weeks.
Week 16
Fetus is 11.5 to 13.5 cm in length. Fetus is 100 g.
Active movements are present. Fetal skin is transparent.
Lanugo hair begins to develop. Skeletal ossification occurs.
Sex of fetus is visually recognizable on ultrasound.
Week 20
Fetus is 16 to 18.5 cm in length. Fetus is 300 g.
Lanugo covers the entire body. Fetus has nails.
Muscles are developed. Enamel and dentin are depositing.
Heartbeat is detected by regular (nonelectronic) fetoscope.
Week 24
Fetus is 23 cm in length. Fetus is 600 g.
Hair on head is well formed.
Skin is reddish and wrinkled.
Reflex hand grasp functions are present.
Vernix caseosa covers entire body.
Fetus has ability to hear.
Week 28
Fetus is 27 cm in length.
Fetus is 1100 g. Limbs are well flexed.
Brain is developing rapidly.
Eyelids open and close.
Lungs are developed sufficiently to provide gas exchange (lecithin forming).
If born, neonate can breathe at this time.
Week 32
Fetus is 31 cm in length.
Fetus is 1800 to 2100 g.
Bones are fully developed.
Subcutaneous fat has collected. Lecithin-to-sphingomyelin (L/S) ratio is 1.2:1.
Week 36
Fetus is 35 cm in length.
Fetus is 2200 to 2900 g.
Skin is pink and body is rounded.
Skin is less wrinkled. Lanugo is disappearing.
L/S ratio is greater than 2:1.
Week 40
Fetus is 40 cm in length.
Fetus is more than 3200 g.
Skin is pinkish and smooth.
Lanugo may be present on upper arms and shoulders.
Vernix caseosa decreases.
Fingernails extend beyond fingertips.
Sole (plantar) creases run down to the heel.
Testes are in the scrotum.
Labia majora are well developed.
Preembryonic Period
First 2 weeks after conception
Embryonic Period
Beginning day 15 through approximately week 8 after conception
Fetal Period
Week 9 after conception to birth
Week 1
Blastocyst is free-floating.
Weeks 2 to 3
Embryo is 1.5 to 2 mm in length.
Lung buds appear. Blood circulation begins.
Heart is tubular and begins to beat.
Neural plate becomes brain and spinal cord.
Week 5
Embryo is 0.4 to 0.5 cm in length.
Embryo is 0.4 g. Double heart chambers are visible.
Heart is beating. Limb buds form.
Week 8
Embryo is 3 cm in length.
Embryo is 2 g. Eyelids begin to fuse.
Circulatory system through umbilical cord is well established.
Every organ system is present.
Week 12
Fetus is 6 to 9 cm in length. Fetus is 19 g.
Face is well formed. Limbs are long and slender.
Kidneys begin to form urine. Spontaneous movements occur.
Heartbeat is detected by Doppler transducer between 10 and 12 weeks.
Week 16
Fetus is 11.5 to 13.5 cm in length. Fetus is 100 g.
Active movements are present. Fetal skin is transparent.
Lanugo hair begins to develop. Skeletal ossification occurs.
Sex of fetus is visually recognizable on ultrasound.
Week 20
Fetus is 16 to 18.5 cm in length. Fetus is 300 g.
Lanugo covers the entire body. Fetus has nails.
Muscles are developed. Enamel and dentin are depositing.
Heartbeat is detected by regular (nonelectronic) fetoscope.
Week 24
Fetus is 23 cm in length. Fetus is 600 g.
Hair on head is well formed.
Skin is reddish and wrinkled.
Reflex hand grasp functions are present.
Vernix caseosa covers entire body.
Fetus has ability to hear.
Week 28
Fetus is 27 cm in length.
Fetus is 1100 g. Limbs are well flexed.
Brain is developing rapidly.
Eyelids open and close.
Lungs are developed sufficiently to provide gas exchange (lecithin forming).
If born, neonate can breathe at this time.
Week 32
Fetus is 31 cm in length.
Fetus is 1800 to 2100 g.
Bones are fully developed.
Subcutaneous fat has collected. Lecithin-to-sphingomyelin (L/S) ratio is 1.2:1.
Week 36
Fetus is 35 cm in length.
Fetus is 2200 to 2900 g.
Skin is pink and body is rounded.
Skin is less wrinkled. Lanugo is disappearing.
L/S ratio is greater than 2:1.
Week 40
Fetus is 40 cm in length.
Fetus is more than 3200 g.
Skin is pinkish and smooth.
Lanugo may be present on upper arms and shoulders.
Vernix caseosa decreases.
Fingernails extend beyond fingertips.
Sole (plantar) creases run down to the heel.
Testes are in the scrotum.
Labia majora are well developed.
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