Fetal Development Notes

Fetal Development: Study Notes

Time Span of Fertilization

• Ovum released from ovary; passes into open fallopian tube; starts journey downward toward the sperm for fertilization.
• Sperm from the male is deposited into the vagina and swims approximately 7\text{ inches} to meet the ovum at the outermost portion of the fallopian tube, the area where fertilization takes place.
• This process occurs in about an hour.
• When one spermatozoon penetrates the ovum’s thick outer membrane, pregnancy begins.
• All this activity takes place within a time span of 5\text{ hours}.

Stages of Fetal Development

• Zygotic stage: fertilization of sperm and egg (conception).
• Blastocyst stage: zygote divides into a solid ball of cells and attaches to uterus.
• Embryonic stage: from the end of the second week through the eighth week; Basic structures of major body organs and main external features form.
• Fetal stage: differentiation and structures specialize by the end of the eighth week until birth.

Embryonic Layers

• Ectoderm: forms the central nervous system, special senses, skin, and glands.
• Mesoderm: forms skeletal, urinary, circulatory, and reproductive organs.
• Endoderm: forms respiratory system, liver, pancreas, and digestive system.

Embryo and Early Development by Month (Weeks 1–4)

• Month 1 (weeks 1 through 4):
  • Your body slowly releases more hormones; the uterus prepares for a potential pregnancy.
  • At the end of the second week, ovary releases an egg (ovulation). If sperm meets an egg just after ovulation, the process to pregnancy continues.
  • Week 3: Fertilization occurs; a sperm and egg join to form a zygote.
  • Week 4: The tiny bundle of cells turns into a blastocyst and implants into the uterine lining. The beginning of what will become the placenta forms. A water-tight sac forms around the blastocyst—the amniotic sac—which cushions the fetus during pregnancy.
• Week 4 note: Yolk sac forms around Week 4.

Month 2 (weeks 5 through 8)

• The second month is when most people realize they’re pregnant.

• Pregnancy hormones go into overdrive; by about the 5th week, an at-home pregnancy test will show positive.

• This is when many people begin to feel symptoms of pregnancy.

• Week 5:

  • The neural tube (brain, spinal cord, and other neural tissue of the CNS) forms.
  • The tiny “heart” tube will beat 110\,\text{bpm} by the end of the fifth week.

• Week 6:

  • Tiny buds that become arms and legs develop.
  • Blood cells form; circulation begins.
  • Structures that’ll become ears, eyes, and mouth take form.
  • Heartbeat detectable on a vaginal ultrasound.

• Week 7:

  • Bones begin replacing soft cartilage; genitals begin to form.
  • The embryo’s head is large in proportion to the rest of the body; resembles a small tadpole or seahorse due to its prominent tail (which becomes legs).

• Week 8:

  • All major organs and body systems are developing.
  • Fetus has web-like hands and feet; eyes become visible; ears begin to form.
  • The umbilical cord is fully developed and helps transport oxygen and blood to the embryo.
  • Fetal heart sounds are heard.

Month 3 (weeks 9 through 12)

• The third month marks the transition from embryo to fetus; rapid growth and development.

• The fetus develops distinct facial features, limbs, organs, bones, and muscles.

• By the end of the 12th week, the fetus has an assigned sex, but it won’t be visible on ultrasound for several more weeks.

• Week 9:

  • Beginnings of teeth and taste buds are forming.
  • Muscles are forming; body takes on more human shape; head is still ~50% of length.
  • Heartbeat may be heard with a Doppler.

• Week 10:

  • Arms, hands, fingers, feet, and toes are fully formed (no webbing).
  • Fingernails and toenails begin to develop; external ears form.
  • External genitals begin to form but too soon to see on ultrasound.

• Week 11:

  • Fetus explores by opening/closing fists and mouth.
  • Knees, elbows, and ankles work; too soon to feel kicks.
  • Bones harden; skin is still translucent.
  • Facial features become more prominent.

• Week 12:

  • All organs, limbs, bones, and muscles are present and continue to develop to become fully functional.
  • Circulatory, digestive, and urinary systems are working; the liver produces bile.
  • Fetus drinks and urinates amniotic fluid.

• Week 12 length: 2.5\text{ to }3\text{ inches} (about a plum).

Month 4 (weeks 13 through 16)

• Many people begin to show; heartbeat audible via Doppler ultrasound.

• Fetus can suck thumb, yawn, stretch, and make faces.

• Week 13:

  • Vocal cords form; the head grows to be proportionate to the body.

• Week 14:

  • Skin thickens; fine hair (lanugo) begins to grow.
  • Fetus can bring fingers to mouth and turn head.
  • External genitals fully developed; fingerprints begin to form.

• Week 15:

  • Some organs (e.g., intestines and ears) move to permanent locations.
  • Lungs are beginning to develop; fetus practices breathing with amniotic fluid.
  • Fetus makes more purposeful movements (thumb-sucking, facial expressions).

• Week 16:

  • Fetus has lips; ears are developed enough to hear speech.
  • Eyes are closed but can react to light by turning away.

• Week 16 length: 5\text{ inches}; weight 4\text{ ounces} (about the size of an avocado).

Month 5 (weeks 17 through 20)

• By the end of the fifth month, most people begin to feel the fetus moving (quickening).

• First ultrasound; sex determined.

• Week 17:

  • Skin still thin but will start to gain fat; skin coated with vernix caseosa (a whitish coating) to protect from amniotic fluid.

• Week 18:

  • Fetus is covered in lanugo (fuzz); sleep-wake cycle may be present; loud noises may wake the fetus.

• Week 19:

  • Fetus grows stronger; kicks are felt; unique fingerprints; hiccups.

• Week 20:

  • Nails grow toward the ends of fingers; brain areas for senses begin to develop.

• Week 20 length: 9\text{ to }10\text{ inches}; weight 1\text{ pound}.

Month 6 (weeks 21 through 24)

• Fetal skin is reddish, wrinkled, and veins are visible through translucent skin.

• In the sixth month, eyelids begin to part; regular, jerky movements occur.

• Fetus responds to sounds by moving or increasing its pulse.

• Week 21:

  • Limb movements become coordinated and frequent; bone marrow develops to produce blood cells.

• Week 22:

  • Grasp strengthens; fetus can touch its ears and the umbilical cord.
  • Fetus can hear heartbeat, stomach rumble, and breathing.

• Week 23:

  • If born prematurely, the fetus may survive after the 23rd week with intensive care; rapid fat accumulation begins.

• Week 24:

  • Lungs are fully developed but not yet ready to function outside the uterus.

• Week 24 length: 12\text{ inches}; weight 2\text{ pounds}.

Month 7 (weeks 25 through 28)

• Fetus continues to mature and develop fat reserves.

• Fetus changes position frequently; responds to stimuli (sound, pain, light).

• Amniotic fluid begins to diminish.

• Week 25:

  • More body fat reduces wrinkling; nervous system rapidly matures.

• Week 26:

  • Fetus produces melanin (skin/eye color); lungs begin making surfactant to aid breathing after birth.

• Week 27:

  • Fetus can open eyes and blink; eyelashes present.

• Week 28:

  • Fetus may begin turning head-down in preparation for birth.
  • End of the seventh month: length 14\text{ to }15\text{ inches}; weight 2\text{ to }3\text{ pounds}.

Month 8 (weeks 29 through 32)

• Fetus continues to mature and accumulate fat reserves.

• Brain develops rapidly during this period.

• Fetus can see and hear most stimuli.

• Most internal systems are well developed, but the lungs may still be immature.

• Week 29:

  • Kicks and jabs feel more like pokes as space tightens.

• Week 30:

  • Fetus can regulate its own body temperature; brain matures rapidly.

• Week 31:

  • Fetus processes more information and stimuli; clearer awake/sleep patterns.

• Week 32:

  • Skin loses translucence; aside from lungs and brain, most organs are well formed and ready for birth.

• Week 32 length: 17\text{ to }18\text{ inches}; weight 5\text{ pounds}.

Month 9 (weeks 33 through 36)

• Fetus continues to grow and mature; lungs near full development.

• Ninth month focuses on finishing growth and brain development.

• Week 33:

  • Bones harden except for brain, which remains soft to allow passage through birth canal.

• Week 34:

  • Vernix thickens to protect skin.

• Week 35:

  • Brain continues growth but weighs about two-thirds of what it will at birth.

• Week 36:

  • Lanugo is shed; head hair present.

• Week 36 length: 17\text{ to }19\text{ inches}; weight 6\text{ to }7\text{ pounds}.

Month 10 (Weeks 37 through 40)

• In this final month, labor could begin at any time.

• Fetus’s position may change to prepare for birth; ideally head-down in the uterus.

• May feel very uncomfortable as the fetus descends into the pelvis (drop) and prepares for birth.

• Providers may encourage kick counts to track fetal movement.

• Week 37:

  • Toenails reach the ends of the toes; may start to drop into the pelvis.

• Week 38:

  • Fetus gains about 0.5\text{ pounds} per week to reach final size.

• Week 39:

  • Fetus is full-term and ready to meet the world.

• Week 40:

  • Due date week; call provider if signs of labor appear.

• Week 40 length: 18\text{ to }20\text{ inches}; weight 7\text{ to }9\text{ pounds}.

Functions of the Placenta

• Serves as the interface between the mother and fetus.
• Produces hormones to regulate maternal physiology to ensure the fetus gets nutrients and oxygen for growth.
• Protects the fetus from immune attack by the mother.
• Removes waste products from the fetus.
• Induces the mother to bring more food to the placenta.
• Produces hormones that mature into fetal organs.

Hormones Produced by the Placenta

• Chorionic gonadotropin.
• Prolactin.
• Human placental lactogen (hPL) or human chorionic somatomammotropin (hCS).
• Estrogen.
• Progesterone.
• Relaxin.

Umbilical Cord

• Formed from the amnion.
• Lifeline from the mother to the growing embryo (fetus).
• Contains one large vein and two small arteries.
• Wharton jelly surrounds the vein and arteries to prevent compression.
• At term, the average umbilical cord is 22\text{ inches} long and about 1\text{ inch} wide.

Role of Amniotic Fluid

• Helps maintain a constant body temperature for the fetus.
• Permits symmetric growth and development.
• Cushions the fetus from trauma.
• Allows the umbilical cord to be relatively free of compression.
• Promotes fetal movement to enhance musculoskeletal development.
• Fluctuates during pregnancy.

Fetal Circulation (Overview)

• Blood flows from the placenta to the fetus and back to the placenta.
• Three shunts during fetal life:
  • Ductus venosus: connects the umbilical vein to the inferior vena cava (bypasses the liver).
  • Ductus arteriosus: connects the main pulmonary artery to the aorta (bypasses the lungs).
  • Foramen ovale: an opening between the right and left atria (bypasses the right ventricle).

Fetal Circulation Diagram Summary

• The diagram shows pathways from placenta to fetus and to placenta.
• Pathways include the umbilical vein, portal vein, inferior vena cava, right atrium, foramen ovale, right ventricle, left atrium, aorta, and connections through the ductus arteriosus and ductus venosus, illustrating how blood bypasses the non-functioning fetal lungs and liver.