Fetal Development
Week 1
The ovum (egg) becomes fertilized in the Fallopian Tube, divides, and travels down this tube to implant into the uterine lining.
Maternal Changes: The ovaries increase production of progesterone, the “pregnancy-maintaining” hormone.
Week 2
The three primitive germ layers are formed-ectoderm, endoderm and mesoderm. These three layers will generate every organ and tissue in the baby’s body.
Maternal Changes: Period is missed.
Weeks 3 and 4
Primitive body segments appear which will form the spine, brain and spinal cord. Digestive tract, heart and blood circulation take shape. The embryo is 1/5” long.
Maternal Changes: The placenta is growing and covers 1/15th of the uterine interior. Breast tenderness begins (for some women, this tenderness can begin as early as 1 or 2 weeks post-conception).
Weeks 5 and 6
The heart starts pumping. Limb buds appear. Eyes begin taking shape. External ears are developing. Major divisions of the brain are discernable.
Maternal changes: There is an exchange of nutrients and oxygen across the placenta. The two circulations-maternal and fetal-remain completely separate.
Weeks 7 and 8
The face is complete with eyes, nose, lips and tongue. Tiny muscles and bones have developed beneath the thin skin. The tiny heart beats about 120-160 beats per minute. First movements start. The embryo is approximately 1 1/8” long and weighs about 1/3 oz.
Maternal Changes: The placenta covers approximately 1/3rd of the uterine lining.
Weeks 9 and 10
The eyelids are formed and sealed shut. Blood cells and bone cells form. Gender can be determined. The embryo is now a fetus.
Maternal Changes: Maternal blood volume has increased by 30-40%.
Weeks 11and 12
The genitalia are fully formed. There is early organ functioning. The kidneys produce urine and the pancreas produces insulin. The lungs have developed and primitive breathing motions begin. The swallow reflex is developed. The fetus is approximately 3” long and weighs about 1 oz. The fetal heartbeat can easily be heard by a doppler.
Maternal Changes: The placenta is functionally mature: The woman experiences a 2-3 lb. weight gain (and so do the husbands sometimes!).
Weeks 13 and 14
Fetal blood vessels develop rapidly. Maturation of the musculoskeletal system has occurred. There is rudimentary control of the body by the nervous system.
Maternal Changes: The woman is beginning to “show” abdominally. She commonly adds approximately 3-5 lbs. in maternal weight gain.
Weeks 15 and 16
The fetus kicks. All fetal organs and structures are formed. The fetus weighs approximately 7 oz. and is about 6”-7” long.
Maternal Changes: The placenta is producing estrogen.
Weeks 17 and 18
Vernix-an oily, cream cheese-like coating covers the fetus’ skin in utero and protects it. Lanugo-a fine, downy hair covers the body and keeps the vernix on the skin. Eyebrows appear.
Maternal Changes: The woman experiences and additional 4-5 lb. weight gain.
Weeks 19 and 20
Eyelashes and eyebrows are developed as well as is hair on the fetus’ head. The fetus weighs approximately ½-1 lb. and is about 10”-12” long.
Maternal Changes: Colostrum is present in the breasts. The uterus is now at the lever of the umbilicus (belly button). The placenta covers one-half of the uterine lining. There is approximately 2 cups (400 ml.) of amniotic fluid present in the amniotic sac.
Weeks 21 and 22
The skeleton is developing rapidly.
Maternal Changes: There is an additional 4-5 lb. weight gain.
Weeks 23 and 24
Eyelashes are formed. Fingernails now extend to the ends of the fingers. The fetus weighs approximately 27 oz. and is about 11”-14” long.
Maternal Changes: The mother can sense when the baby is awake.
Weeks 25 and 26
The fetus’ eyelids are open and close. He/she makes respiratory movements and can swallow. The fetus can also regulate its body temperature from its own hypothalamus but still depends upon maternal support.
Maternal Changes: There is an additional 4-5 lb. weight gain.
Weeks 27 and 28
The fetus is 2/3rds grown with its weight about double that at age 24 weeks. The fetus is approximately 14”-17” long.
Maternal Changes: The mother can detect fetal hiccups.
Weeks 29 and 30
Fat is being deposited under the fetal skin in order to insulate the fetus in preparation for the abrupt temperature changes which occur at birth.
Maternal Changes: There is an additional 4-6 lb. weight gain.
Weeks 31 and 32
The fetal skin is less red and wrinkled. Its lungs and digestive tract are almost fully mature. The bones of the head are soft and flexible. The fetus is approximately 16”-18” long.
Maternal Changes: The mother may have varying degrees of insomnia due to the fetus’ activity.
Weeks 33 through 36
The fetus takes up most of the uterine space and its movements are restricted. Most fetus’ turn into a vertex-head down-position during this time.
Maternal Changes: The placenta weight about 20 ozs.
Weeks 37 through 40
The fetus’ fingernails may protrude beyond the ends of its fingers. The fetus weighs approximately 6-9 lbs. and measures about 18”-22” long. Maternal antibodies are transferred across the placenta to the baby providing protection for approximately 6 months.
Maternal Changes: There may be an additional weight gain of 2-6 lbs. The baby “drops”, or descends, deeper into the mother’s pelvis in preparation for labor and birth.
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