On the first menstrual day a new reproductive cycle begins. The endometrium (the mucosa that surfaces the internal cavity of the uterus) exfoliates itself and prepares to grow again. In a 28 days cycle, the first 14 days are called “follicular phase”, when the ovaries produce the hormone estradiol, which stimulates the endometrial growth. At mid cycle, ovulation occurs: the LH peak (a gonadotropin produced by the hypophysis) stimulates the release of the mature egg. The ruptured follicle becomes the corpus luteum, which produces estradiol and progesterone. The second 14 days of the cycle are called “luteal phase”: the endometrium becomes thicker and gets prepared for an eventual embryo implantation. If pregnancy does not occur, after 14 days, the corpus luteum ceases its function and regresses. The endometrium, no more stimulated by progesterone, exfoliates itself and a new menstrual cycle begins. The cycle is usually 28 days long, but it is normal to have periods between 25 and 35 days. In these cases, the follicular phase is variable in length, while the luteal phase is always 12-14 days (which is the lifespan of the corpus luteum).
The egg and the sperm meet in the fallopian tube, the embryo is transferred to the uterus, where it implants approximately 7 days after conception. The trophoblast, the external cellular mass which will become the placenta, informs the mother of its presence through the hormone beta-hCG, which stimulates the corpus luteum to continue progesterone secretion, essential to maintain the pregnancy.
The days around ovulation are potentially fertile. The sperm may live up to 5-6 days in the female genital organs, meaning an intercourse occurring 4-5 days before ovulation may be fertile. Otherwise, the egg lives only 24 hours, so two days after ovulation, the fertile window has finished. It is enough to have two intercourses a week to be sure to cover the fertile window.
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