occurs when a sperm's chromosomes combine with those or an egg (sec oocyte) to form a fertilized egg, or zygote.
6-7 days after ovulation this begins. the window of implantation is opened by the surging level of ovarian hormones (estrogen & progesterone) in the blood. (1076)
the placenta is usually fully functional as a nutritive, respiratory, excretory, and endocrine organ by the end of the 3rd month of pregnancy. throughout pregnancy, blood levels of estrogens and progesterone continue to increase. they encourage gowth andfurther differentiation of the mammary glands and ready them for lactation. the placenta also produces human placental lactogen, human chorionic thyrotropin, and relaxin (1078)
hormones during pregnancy
relaxin, human placental lactogen (hPL)/ human chorionic somatomammotropin (hCS), human chorionic thyrotropin (hCT)
relaxin: causes pelvic ligaments and the pubic sumphysis to relax, widen, and become more flexible
human placental lactogen (hPL/hCS)
works with estrogens and progesterone to stimulate maturation of the breasts for lactation, promotes growth of the fetus, and exerts a glucose-sparing effect in the mother.
human chorionic thyrotropin (hCT)
a glycoprotein hormone similar to the thyroid-stimulating hormone. it increases the rate of maternal metabolism throughout the pregnancy, causing hypermetabolism.
stages of developement of the fetus
initiation of labor
during last few weeks, estrogens reach highest levels in mother's blood. rising levels of adrenocortical hormones(cortisol) late in pregnancy are a major stimulus for the placenta to release such large amounts of estrogen. rise in estrogens causes (1) antagonizes preogesterone's quieting influence on uterine muscle-resulting in the myometrium becoming irritable, weak, and irregular uterine contractiong occur (false labor). As birth nears,2 more chemical signals cooperate to convert these false labor pains into the real thing. certain fetal cells begin to produce OXYTOCIN which causes the placenta to release prostaglandins. both are uterine muscle stimulants,contraction become more vigorous and frequent. drop of progesterone (1091)
production of milk by the hormone-prepared mammary glands. rising levels of (placental) estrogens, progesterone, and himan placental lactogen toward the end of pregnancy stimulate the hypothalamus to release prolactin-releasing factors. the anterior pituitary gland responds by secreting prolactin. (1093)