A woman has two almond-shaped and sized ovaries (one each side of the uterus). They take turns each monthly cycle to produce and develop eggs (ova) for fertilization.
The egg journey begins in an ovarian follicle. As many as a dozen follicles, which are egg-containing, ¼"-1" diameter cyst-like sacs filled with fluid and hormones (mostly estrogen), start to ripen within an ovary during the first 14 days of the menstrual cycle (which begins day 1 of the monthly period). One or two of these follicles grow stronger than the others, producing and maturing an egg. When ovulation is triggered by a surge in LUTEINIZING HORMONE (LH), the dominant follicle ruptures, releasing its egg into the fallopian tube en route to the uterus, where either the egg is fertilized by a sperm within 24 hours, or it dies.
The now empty follicle egg sac in the ovary becomes a corpus luteum, producing and secreting ESTRADIOL and PROGESTERONE to prepare and provide support for a possible pregnancy. PROGESTERONE is produced from yellow, fatty cholesterol in the corpus luteum, which is Latin for "yellow body". It's pro-gestational functions include:
• Thicken the uterine lining (endometrium) to a max. 4-6 mm
• Aid secretion of nutrients from endometrial glands
• Stimulate secretion of immune system Th2 and reduce secretion of Th1 cytokines. To prevent mother's immune system from responding to fetal antigens and rejecting the newly arrived "foreign body" :)
Under the influence of HUMAN CHORIONIC GONADOTROPIN (hCG ) from a developing embryo - the corpus luteum ovarian granulosa cells produce large amounts (25-50 mg/day) of PROGESTERONE and smaller amounts (0.5mg/day) of ESTRADIOL) crucial for the first 10 weeks of gestation (counted from day 1 of last mentrual period) (Csapo, 1973) , at which time the placenta is usually able to produce sufficient PROGESTERONE to support the pregnancy. The fertilized egg undergoes rapid cell division and implants on the prepared uterine wall 5-6 days after ovulation. hCG must appear 4 days after ovulation to rescue the corpus luteum from regression. Corpus luteum has a negligible role in 2nd and 3rd trimesters.
At term gestation, the placenta produces about 250 mg PROGESTERONE /day and PROGESTERONE levels range from 100-200 ng/ml.
Without any hCG influence from an embryo, then about 10 days after ovulation, the corpus luteum usually disintegrates (via a process called luteolysis). Next, the uterine lining is shed (seen as menstrual bleeding),
References and Resources
Csapo AL, Pulkkinen MO, Wiest WG. (Mar 1973) Effects of luteectomy and progesterone replacement in early pregnant patients. Am J Obstet Gynecol. 115:759. [PubMed]
Pratap Kumar, Navneet Magon. (2012) Hormones in Pregnancy Niger Med. J. Oct-Dec; 53(4): 179-183 Pub Med
