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 4th week (detected by end of the 1st trimester with a stethoscope

• 8th 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 10-12 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