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The single cell, fertilized egg. The zygote reproduces itself through mitosis and enters the uterus in about 3 days.


The inner mass of cells that will develop into the embryo and amnion.


The outer mass of cells that will develop into the chorion or placenta.


7-9 days after fertilization the blastocyst implants itself in the uterine lining. Usually in the upper posterior wall.

Germ layers

Ectoderm, mesoderm & endoderm - differentiate into different body parts.

Yolk sac

Forms 8-9 days after conception, forms RBCs until liver takes over at 5-6 weeks gestation.


It is the 1st membrane to develop. It encloses the embryo, amnion & yolk sac.

Chorionic villi

These are finger like projections that form the fetal portion of the placenta. By the 8th week, chorionic villi sampling is possible.


A thin protective membrane that contains amniotic fluid.

Fraternal twins

Twins that develop from two separate eggs & sperms.

Identical twins

Develop from one fertilized egg. May be one or two placentas, amnions and chorions. Always the same sex!

Amniotic fluid function

To cushion against injury, control temperature, permits symmetric growth, allows freedom of movement, plays a role in lung development.

Amniotic fluid volume

It moves back and forth across placental membranes. The fetus swallows up to 400 ml per day. 600-800 ml flows in and out of the fetal lungs. Fetus adds to volume by excreting urine.


Less than normal amount of fluid - less than 1000 mls

Polyhydramnios or hydramnios

More than normal fluid - greater than 2000 mls of fluid.

Beginning of placental function

Metabolic exchange occurs by the 4th gestational week. Most women aren't aware they're pregnant.

Placental metabolic functions

Provides glycogen, cholesterol and fatty acids to fetus. Produces enzymes needed for fetoplacental transfer. Breaks down epinephrine and histamine.

Placental transport functions

Simple diffusion - high to low concentration. Facilitated transport - carrier molecules. Active transport - from low to high concentration. Pinocytosis - transfer large molecules by being engulfed. Hydrostatic & osmotic pressure - water & solutes.

Placental drug transfer

Substances with a higher molecular weight can't cross. Heparin can't but Coumadin does.

hcG function

It is present 8-10 days after fertilization. Maintains corpus luteum so progesterone levels are adequate to maintain pregnancy. If progesterone level falls prior to 11th week, a spontaneous abortion will occur.

Estrogen in pregnancy

Enlarges the breasts and uterus. Increases vascularity and vasodilatation.

Progesterone in pregnancy

Maintains the pregnancy. Secreted by the placenta after 11th week. Decreases contractility of smooth muscle.

Umbilical cord

Circulatory pathway from chorionic villi to embryo. Contains one vein and two arteries. These are surrounded by Wharton's jelly to prevent compression of cord.

Ductus venosus

Shunt that bypasses the liver, closes when cord is clamped.

Foramen ovale

It provides a one way opening between R & L atria so blood can avoid non-functioning lungs. Closes when pressure in LA becomes higher than RA.

Ductus arteriosis

Shunt that bypasses the lungs, opening between the pulmonary artery & aorta. Changes occur within minutes of birth with complete closure in 10-15 hours.

Funic suffle

Purring sound heard over the uterus and having the same rate as the fetal heart beat.

Pre-embryonic stage

1st 14 days after fertilization - placental exchange begins at 14 days.

Embryonic phase

15 days to 8th week.

Fetal phase

From end of 8th week to delivery.

Embryonic phase development

Differentiation of tissues into organs, heart begins to beat at 3 to 4 weeks, sexual differences established, but not visible, most vulnerable to teratogens. At the end of 8 weeks, all body organs formed and clearly resembles human being.

9-12 weeks

Fetal heart tones can be heard with doppler. Sexual differentiation is complete, but still not detectable by ultrasound. Palmer grasp is present.

13-16 weeks

Lanugo begins do develop. Sex is detectable by ultrasound.

20 weeks

Quickening first felt by mother. Heartbeat audible with fetoscope, not a stethoscope. Entire body covered with lanugo.

24 weeks

Eyes are structurally complete, alveoli in lungs begin to form - important for preterm infant.

25-28 weeks

Rapid brain development, most fetuses are viable at 27 weeks with immediate and prolonged intensive care. Surfactant forms at 28 weeks.

29-32 weeks

Gaining weight - subcutaneous fat

36 weeks

Lanugo begins to disappear, fetus has a good chance at survival - still considered preterm.

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