The single cell, fertilized egg. The zygote reproduces itself through mitosis and enters the uterus in about 3 days.
7-9 days after fertilization the blastocyst implants itself in the uterine lining. Usually in the upper posterior wall.
These are finger like projections that form the fetal portion of the placenta. By the 8th week, chorionic villi sampling is possible.
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.
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.
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.
Progesterone in pregnancy
Maintains the pregnancy. Secreted by the placenta after 11th week. Decreases contractility of smooth muscle.
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.
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.
Shunt that bypasses the lungs, opening between the pulmonary artery & aorta. Changes occur within minutes of birth with complete closure in 10-15 hours.
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.
Fetal heart tones can be heard with doppler. Sexual differentiation is complete, but still not detectable by ultrasound. Palmer grasp is present.
Quickening first felt by mother. Heartbeat audible with fetoscope, not a stethoscope. Entire body covered with lanugo.
Eyes are structurally complete, alveoli in lungs begin to form - important for preterm infant.
Rapid brain development, most fetuses are viable at 27 weeks with immediate and prolonged intensive care. Surfactant forms at 28 weeks.