I found this article, pasted below, on Fertility Neighborhood. As women, especially ones who want to be surrogates or egg donors, we often ask what are the "reproductive years"? I hope that this post answers some of your questions. (please note that it was reviewed in 2005)
The Reproductive Years
A woman's reproductive years, approximately ages 15-44, are the years in which she can become pregnant and bear children. The defining characteristic of this stage is the presence of regular menstrual cycles, which generally indicate that a woman is ovulating. Cycles usually become regular within two years of the first menstrual period (menarche).
Reproductive ability peaks around age 29 and then gradually decreases. After age 35, reproductive potential wanes rapidly, and by age 44, the chances of conceiving and bearing a child are slim.
The menstrual cycle
The menstrual cycle is designed to prepare the endometrium (the lining of the uterus) for pregnancy. The average length for a menstrual cycle is 28 days, but individual women can have cycles as short as 25 days and as long as 32 days and still be considered in the range of normal. In general, you can expect your period to begin approximately two weeks from the date of ovulation. In a 28 day cycle, ovulation typically occurs between the 13th and 15th day. Each month's cycle can be divided into three phases. There are two ways to look at these three phases. One way, called the ovulatory cycle, is based on the function of the ovaries, as follows:
In the follicular phase, many oocytes (immature eggs) begin to go through the maturation process. The oocytes are housed in follicles (fluid-filled sacs) on the ovary. By day 5-7, one follicle becomes dominant. The chosen follicle continues to develop, and secretes increasing levels of the hormone estrogen (in a form called estradiol). The follicles that were not selected recede.
In the ovulatory phase, the egg in the dominant follicle completes its final stages of maturation. At midcycle, the increasing estrogen level prompts a surge of luteinizing hormone (LH) from the pituitary. At that signal, the follicle ruptures (breaks open), releasing the egg, which begins traveling toward the uterus.
In the luteal phase, the remnant of the ruptured follicle transforms into the corpus luteum. Instead of estrogen, it now produces the hormone progesterone.
At the same time that the ovulatory cycle is proceeding, the menstrual cycle is also occurring. From this point of view, it's possible to define the three phases of the cycle based on the activity of the endometrium, as follows:
The menstrual phase occurs if there is no pregnancy already growing in the uterus. The uterus sloughs off the mature endometrium, causing a menstrual period.
In the proliferative phase, the endometrium begins to develop and thicken, in response to the estrogen secreted by the developing follicles. The number of glandular cells and surrounding stromal (support or structural) cells in the endometrium increase.
In the secretory phase, the endometrium continues to develop, responding to the progesterone secreted by the corpus luteum in the ovary. The endometrium is mature for eight to nine days after ovulation. If fertilization does not occur, the endometrium begins to regress.
During the luteal phase and the simultaneous secretory phase, one of two things happens. If the egg was fertilized and succeeds in implanting in the uterus, a pregnancy begins. The implanted embryo begins to develop a placenta, the organ that will nourish the fetus during pregnancy. The early placental cells begin to produce a hormone called human chorionic gonadotropin (hCG). This hormone signals the corpus luteum to maintain its progesterone production through the first six to nine weeks of pregnancy. The progesterone, in turn, signals the endometrium to stay in place in the uterus.
Alternatively, if the egg is not fertilized, there is no embryo to implant in the uterus. About 14 days after ovulation, progesterone production by the corpus luteum falls to the point that it no longer sustains the endometrium. The uterus sheds the endometrial tissue, causing a menstrual period. The first day of menstrual bleeding counts as the first day of the cycle.
The 14-day lifespan of the corpus luteum is determined by a preset clock, allowing enough time for implantation to occur. Therefore, the luteal phase is generally a constant 14 days. In women whose cycles are longer or shorter than the typical 28 days, the variation occurs in the first half of the cycle.
The menstrual period
The endometrium consists of two layers. The top layer, called the spongiosum, contains blood vessels, fluid, and tissue. The lower layer, called the basalis, contains special cells that are capable of regenerating the spongiosum. When estrogen and progesterone levels fall at the end of the menstrual cycle, small blood vessels within the endometrial lining go into rhythmic contractions called vasospasm. This separates the two layers. The spongiosum drops off, creating menstrual bleeding, while the basalis remains in place.
Because there is so much variation among individual women, there is no one definition of a normal period. In general, though, menstruation is considered normal when bleeding occurs every 21-35 days and lasts between three and seven days. Menstrual flow normally starts out light and is rusty in color, becoming heavier and brighter red for a few days, then lessening and darkening again toward the end. Typically less than 80 milliliters (about 5 tablespoons) of blood is lost during a typical menstrual period, although it often may appear to be more.
If you have a regular menstrual cycle, that usually means you are ovulating. However, it's possible to have vaginal bleeding that looks like a period without releasing an egg. Determining if and when ovulation is occurring is an essential step in fertility treatment. Your doctor can help you choose the best way to detect ovulation so that you can increase your chances of becoming pregnant.
It is not uncommon for many women to feel ovulation as lower abdominal discomfort in the middle of their cycle. This discomfort, called mittelschmerz, may be due to irritation of the lining of the pelvis and abdomen from the chemical substances released by the ruptured follicle.
Other cycle-related signs and symptoms that are likely to mean you are ovulating include:
• breast tenderness • fluid retention • menstrual cramps • back pain • mood changes • a midcycle increase in cervical mucus • a midcycle increase in your basal body temperature
Reviewed on January 3, 2005 by Gina Paoletti-Falcone, RN, BSN.