The first day of menses is generally considered to be the first day of full flow, which if counting, is considered cycle day 1 (CD1). Many women have spotting days before their full flow with these not being counted as their first day. The primary reason menses occurs is that a pregnancy was not achieved in the last cycle, resulting in a huge drop in hormones (Progesterone and Estrogen) that destabilizes the uterine lining which creates this shedding known as menses.
The ovary during this time is actually already preparing for upcoming ovulation. It has already recruited several follicles (individual sacs which contain eggs) to fight to see which one will ovulate. The follicles at this point are all less than 10mm in size, and can be visualized on ultrasound and given the name antral follciles. A woman under the age of 35 will on average have about 15-20 follicles vying to be the one that ovulates. In women who are over the age of 35, the cohort of follicles decrease on a yearly basis until there are no further follicles and thus ovulation (Menopause)
Follicular Phase: Days 5-8
The strongest follicle of this cohort of 15-20 will be the one that survives this "selection" period, where hormone levels of FSH drop, creating this "weeding out" process. These follicles undergo atresia, or basically cellular death, to ensure that only one survives for ovulation. This process is highly important in humans so that we only have a singleton the majority of times at birth. Many other mammals don't undergo this selection process as effectively, hence they usually have "litters" upon birth due to the fact many follicles underwent ovulation and not atresia.
Follicular Phase: Days 9-14
Follicular dominance by the single surviving follicle is that is committed towards a path of ovulation. People who don't menstruate monthly fail to ovulate, and thus fail to create a dominant follicle. If this is your case, please visit a doctor for an evaluation if more than 3 months have passed without a period and/or you have fewer than 9 periods a year. The dominant follicle at this point will grow rapidly at a rate of 1-3mm/day. As the follicle grows, it produces (via its granulose cells) more and more Estrogen on a daily basis until the production peaks at about 200-250 pg/mL. This is how the ovary communicates with the brain (the pituitary) to trigger ovulation.
The brain recognizes this large level of Estrogen the follicle is secreting, prompting the brain to trigger ovulation through a surge of hormone called LH. Once this surge is detected by the follicle, it will ovulate 36-42 hours later. Of note, it is the Estrogen created that will proliferate/thicken the endometrium so that there's good vascularity for future embryo implantation.
(with regular cycles usually somewhere around CD14)
This is a one time event per menstrual cycle that occurs only after all these complex scenarios. Ovulation is by definition the release of the egg from the follicle. The egg is quickly picked up within seconds by the suction created by the fallopian tube's finger like projections (fimbriae). The egg will only live in the fallopian tube for a few hours before it dies off, and has to be fertilized by sperm almost immediately. This is why we say that it is much better to have the sperm there waiting for the egg since sperm live much longer.
The follicle at this point will fill up with blood vessels and blood creating what is called the corpus luteum, which pound for pound, secretes the highest quantity of hormones out of any gland within the human body (Progesterone and Estrogen).
The Luteal Phase (the day after ovulation until menses)
If the egg is fertilized by sperm and doesn't demise within the first day, then the subsequent embryo will take on average about 5-6 days to travel the fallopian tubes and arrive into the endometrium (Uterine Cavity). The large amounts of Estrogen that was made during the follicular phase created a lush environment for the embryo to thrive (see embryo development below).
The large amounts of Progesterone that is now being created, is what keeps the endometrium stabilized and keeping it from shedding (what is called menses) along with helping with imiplantation. There has been a myth propagated by the internet along with books about implantation spotting/cramping, but the medical community has not recognized that this is truly an actual event as one must remember that an embryo is microscopic and that spotting can also be caused by a myriad of reasons.
It is the Progesterone created during the luteal phase that will make your temperature rise. The temperature should be taken upon opening one's eyes upon awakening. Many recommend having your temperature monitor on a nightstand near you. For a valid result, one should have gotten at least 3 hrs of sleep. The corpus luteum, responsible for the luteal phase, only lives for 14days (+/-2 days) unless rescued by a pregnancy. If there is no pregnancy, then the demise of the corpus luteum is what causes the huge drop in Progesterone and Estrogen which is what creates menses. If there is a pregnancy, then the pregnancy will keep the corpus luteum alive (via hCG) and producing Progesterone until the 10th wk of gestation, when the pregnancy itself will secrete its own Progesterone.
Fertilization must occur within the first day for an embryo to develop. Hence, thereafter in the menstrual cycle, achieving a pregnancy is almost impossible. 24hrs after fertilization, the zygote (the name of the fertilized egg at this point), is still one cell which contains the sperm's DNA and egg's DNA compacted on separate poles of the egg. Each DNA will travel to the center of the egg where they will fuse, creating an embryo consisting of both paternal and maternal features. 48hrs after fertilization, the embryo has divided from one cell into four cells, it is at this point that paternal gene's start kicking in and contributing towards embryo growth and development. 72hrs after fertilization, the embryo has doubled in size and now has 8 cells.
From this point on, rapid cellular growth is encountered with complex embryonic division so that at 5 days after fertilization, hundreds if not thousands of cells have been created from the original one and is now called a blastocyst. It is the blastocyst which finally makes it into the uterine cavity for implantation after its long journey through the fallopian tubes. In order for successful implantation to occur, the blastocyst must burrow itself within the endometrium so that it can receive a rich supply of nutrients from the maternal circulation for the embryo to grow and thrive. After a healthy and successful implantation, the embryo's placenta will secrete hCG which will allow the corpus luteum to continue making essential Progesterone so that the endometrium doesn't shed along with the embryo. After millions of cellular divisions, already highly divided as to what's going to be skin, bone, intestines, uterus, kidneys, etc., a fetal heart beat can be seen via ultrasound after 6wks gestation when further growth of all organs continue to occur until delivery and into early neonatal life.
This information was written by Dr. Rudy Quintero, M.D., F.A.C.O.G. a Board Certified Reproductive Endocrinologist, and a Fertility Doctor in Los Angeles, to read more about him please visit www.CareFertility.net