Conception and Fetal Development

Placenta

Temporary fetal organ formed by 12 weeks that acts as an interface between a pregnant person and fetus, separated by placental barrier

Placenta functions

Provides and allows gas/nutrient/waste exchange, produces hormones that maintains pregnancy (hCG), and protects fetus from immune attacks from mother

Placental barrier

Separates maternal and fetal bloods with substances moving across the barrier via diffusion

Umbilical cord

Connects fetus to placenta, containing umbilical vein (supply fetus), 2 umbilical arteries (lowO2 blood and waste away), and Wharton’s jelly (cushions cord)

Amnion

Inner membrane that surrounds amniotic cavity that contains amniotic fluid (closest to fetus)

Chorion

Outer fetal membrane that surrounds amnion and embryo. Develops chorionic villi that allows exchange of oxygen/waste/nutrients from maternal blood at placenta

Amniotic Fluid

Fluid that surrounds embryo that maintains temperature, cushions fetus and umbilical cord, allows symmetric, MSK, and lung development

Stages of Fetal Development

Preembryonic stage → embryonic stage → fetal stage

Preembryonic stage phases

Fertilization → cleavage → blastocyst → implantation; weeks 1-2

Fertilization

Union of ovum (23) and sperm (23) → zygote w/ 46 chromosomes that occurs in outer third of ampulla of fallopian tube, establishing genetic makeup

Cleavage

Zygote begins rapid mitotic cell division (1→2→4→8→16…) while ball of cells travels down fallopian tubes toward the uterus

Morula

Solid ball of cells formed during cleavage of 16+ cells that travels down to uterine cavity that happens after 72 hrs of fertilization

Blastocyst

Zygote → morula → early embryonic structure that is made up of an inner cell mass and outer cell mass that forms 5 days after fertilization

Inner cell mass (embryoblast)

Becomes embryo and amnion → 3 germ layers (ectoderm, mesoderm, endoderm)

Outer cell mass (trophoblast)

Becomes chorion, helps form most of placenta, and implants itself into endometrium

Ectoderm

Forms central nervous system, special senses, skins and glands

Mesoderm

Forms skeletal, urinary, circulatory, and reproductive organs

Endoderm

Forms respiratory system, liver, pancreas, and digestive system

TORCH infections

Toxoplasmosis, Other (varicella), Rubella, Cytomegalovirus, Herpes Simplex, Listeriosis

What does SAB mean?

Spontaneous abortion

Medications that can’t be taken during pregnancy

Antiepileptics, large vitamin A doses, anticoagulants, tetracycline, mercury-rich substances

When is the fetal stage?

Weeks 9—37-40

Fetal circulation

No lungs for gas exchange and livers unable to process blood, so placenta and shunts redirects blood around fetus’s immature lungs and liver for efficiency

Ductus venosus

Shunts oxygenated blood from umbilical vein → inferior vena cava, bypassing most of liver

Foramen ovale

Shunts oxygenated blood from right atrium → left atrium, bypassing the lungs and skipping gas exchange

Ductus arteriosus

Shunts oxygenated blood by connecting the pulmonary artery → aorta, bypassing the lungs

Why are weeks 2-8 during embryonic stage so important?

Major organs began to form (organogenesis) and embryo very vulnerable to teratogens that may cause birth defects

Week 3

Neural tube forms and eventually becomes brain and spinal cord; yolk sack provides nutrients and o2 until placenta

Weeks 4-8

Brain and spinal cord develops, heart starts beating, eyes ears limbs and facial structures begin forming

Oligohydramnios

Too little amniotic fluid from renal problems, ruptured membrane, bad placenta → umbilical cord compression, poor lung development, abnormal positioning

Polyhydramnios

Too much amniotic fluid from fetal swallowing problems to recycle (GI obstruction) → uterine overdistention, premature rupture, ↑ postpartum hemorrhage risk, preterm labor, malpresentation

Weeks 9-12

Embryo is now fetus, head is large, kidneys produce urine, distinguishable gender genital

Weeks 13-16

Lanugo (soft fine hair) covers fetus and protects skin; quickening is detected by mother (fetus already moving prior)

Weeks 17-20

Fetus has vernix or protective creamy substance on skin, heart tones can be heard more easily with scope, more facial features

Weeks 21-24

Hand grasp reflex and startle reflex (ears develop); Alveoli form for gas exchange after birth and surfactant, but lungs still immature, eyes sealed

Weeks 25-28

Eyelids can open and close, fetus in head-down position, fetal thumb sucking (dev. of coordination)

Weeks 29-32

Fetus accumulates more body fat (growth, energy storage, temperature regulation after birth) and rhythmic breathing movements (practicing, not actual air yet)

Weeks 33-38

Lanugo begins disappearing, strengthened grasp reflex shows nervous and muscular development, testes descend into scrotum, lungs almost fully mature