Reproduction in women
The female reproductive system is entirely internal.
The vagina is the canal that leads from the outside of the body to the cervix, the opening to the uterus.
The uterus is the muscular organ where a fertilised egg, or embryo, attaches and develops. It is the size and shape of a pear and lined with a rich and nourishing membrane, the endometrium.
The fallopian tubes extend from the top of the uterus down over the ovaries, the two walnut-sized organs that contain the eggs.
The eggs in each ovary are made before a woman is born. The most eggs a woman will ever have - about 7 million - is when there's still 20 weeks to go before birth. From this time on, the number of eggs will diminish in number, and none will be replaced. A girl is born with about 2 million eggs. At the time she has her first period there are about 400,000.
Every month from puberty to menopause, eggs begin to mature inside several fluid filled 'cysts' within the ovaries, called follicles. Only one of these follicles will become dominant, while the others will shrink and be absorbed by the ovary. At mid cycle, the dominant follicle releases a single egg during ovulation, which then travels down the fallopian tube toward the uterus.
For a pregnancy to occur, fertilisation happens in the fallopian tube, when the egg meets sperm. The developing embryo then travels down the fallopian tube to the uterus, where it will implant in the endometrium approximately 7 days after ovulation.
Hormones control the highly complicated sequence of events leading to ovulation. The pituitary gland in the brain produces the two hormones that the ovary needs:
- Follicle stimulating hormone (FSH)
- Luteinising hormone (LH)
As these hormones are released, the monthly menstrual cycle occurs in three phases:
Phases of the menstrual cycle:
- Follicular phase - this begins on Day 1 of the menstrual cycle, the first day of menstrual bleeding. FSH begins by stimulating the development of many follicles, but as levels of FSH gradually fall in a natural cycle only one follicle will dominate and go on to produces a mature egg. The non-dominant follicles are absorbed by the ovary and cannot be used again. The developing follicle also secretes estrogen, which has several functions. For example, estrogen develops the watery mid cycle changes in cervical mucus that assist the passage of sperm into the uterus and also causes the thickening of the endometrium required for implantation. The main estrogen the ovary produces is estradiol (E2).
- Ovulatory phase - this phase is short. It begins when, in response to rising estrogen levels, the level of LH rises dramatically. This LH surge triggers the final maturation of the egg, the rupture of the follicle, and then the release of the egg. This usually happens 14 days before the next period is due, or on day 14 of a 28-day cycle.
- Luteal phase - begins after ovulation. At this point, the ovarian follicle where the egg developed collapses and solidifies to become the corpus luteum. This very important structure mainly produces progesterone, the hormone necessary for transforming the endometrium so that a fertilised egg (the early embryo) can implant and develop. If conception does not occur, the corpus luteum stops functioning on about Day 26 of a 28 day cycle. Without the support of progesterone, the endometrium begins to break down and is shed in menstruation.