CONCEPTION, DEVELOPMENT, AND BIRTH CHAPTER 46 (AP BOOK)

Pregnancy (gestation) is the condition of carrying one or more embryos in the uterus –Human: last days (38 weeks) from fertilization –Correlates to body-size and maturity of offspring at birth Rodents: 21 days, elephants: 600 days

Conception Fertilization of an egg by a sperm –Occurs in oviduct, 24hrs zygote starts dividing Cleavage: when zygote begins to divide –Embryo becomes a ball of cells by the time it reaches the uterus 3-4 days later –1 week after fertilization: Blastocyst: A sphere of cells containing a cavity –After several days, the Blastocyst implants into the endometrium –Embryo secretes hormones that signal its presence Human chorionic gonadotropin (HCG) –Acts like pituitary LH –Without this there would be a decline in maternal LH due to inhibition of the pituitary »Results in menstruation and loss of the embryo –High levels in urine, used for pregnancy test

First Trimester (about 3 months) Most radical change for baby and mother Endometrium responds to implantation by growing over blastocyst –Differentiation of embryo’s body structure begin –During first 2-4 weeks, embryo get nutrients directly from endometrium Trophoblast (outer layer of blastocyst) grows and combines with endometrium to create the placenta –Placenta: disked-shaped organ, containing embryonic and maternal blood vessels –Grows to about the size of a dinner plate and weighs about 1kg »Diffusion: umbilical cord

First trimester cont. Main period of organogenesis –Development of body organs Heart begins beating by 4 th week (detected by end of the 1 st trimester with a stethoscope 8 th week all major adult structures are present in rudimentary forms –Most susceptible to threats: radiation and drugs After 8 weeks the embryo is called a fetus (only 5 cm long)

Second Trimester Pregnancy becomes obvious Fetus grows to about 30 cm, very active Hormone levels stabilize as HCG declines The corpus luteum deteriorates and placenta takes over production of progesterone, maintains pregnancy

Third Trimester Baby is now about 50 cm long (weighs, 3-4 kg) Activity decreases, fetus is filling most of the space Going into labor: –Estrogen (highest level) includes formation of oxytocin receptors on uterus –Osytocin is produced by the fetus and mothers posterior pituitary gland Stimulates powerful contraction caused by smooth muscles in uterus Stimulates placenta to secrete prostaglandins enhance contractions (positive feedback)

Birth Also called parturition –Brought about by strong rhythmic uterine contractions –3 stages: Opening and thinning of the cervix, complete dilation Expulsion or delivery Delivery of placenta (normally follows baby) Lactation: decrease in progesterone free up anterior pituitary to allow prolactin secretion 2-3 days delay –Release of milk controlled by oxytocin

In Vitro Fertilization Assisted reproductive technology Drugs are injected once a day for 3 weeks to stop woman's menstrual cycle Large doses of FSH are injected once a day for days HCG is injected 36 hours before egg collection, to loosen the egg in the follicles an to make them mature Man provides sperm by ejaculating into a jar –Its processed to concentrate the healthiest ones Eggs are removed from woman’s ovaries –Oocytes are mixed with sperm in culture dishes and incubated for several days –Once they are at least 8 cells, the embryos are inserted into the woman’s uterus

Ethical issues with IVF Against Inherited forms of infertility Denies some embryos a chance at life Laboratory made vs. love Infertility should be accepted as will of GOD Affirmative Infertility due to environment Embryos that are killed cant feel pain Reduce genetic diseases by screening Loving parents b/c its expensive Brings happiness to couples