As a woman ages, unrecognized miscarriages become more frequent. It is very difficult to study this reproductive loss. However, some researchers have used a peculiar phenomenon called the "vanishing twin" to investigate this fairly common event. One-fifth (some researchers have claimed up to 78 percent) of all women who have conceived twins end up delivering only one baby. Where does that second baby go? One explanation is the mother miscarries one of the twins and doesn't know it, thus the term "vanishing twin" (6). Unrecognized miscarriages of single fetuses are probably just as common.

There are other reproductive changes that adversely affect a woman's fertility the older she gets. Estrogen, needed to promote menstruation and maintain pregnancy, begins to decrease as early as 35 years old of age. Older women have less fertile cervical mucus, required to support sperm waiting for or traveling to an egg (7). Less fertile mucus means a woman is less fertile.

Fertility Signs
While a man is fertile every day of the month, a woman can conceive about one-third or less of the time. The average egg, once it escapes from the ovary, only lives for 12 to 24 hours. Sperm can survive for up to five days when they are protected by fertile mucus. But if they enter the vagina during a woman's infertile time, they are dead within four hours and as soon as half an hour.

Fertility awareness, a system that records daily changes in a woman's basal body temperature, cervical mucus, and the position and quality of the cervix, is one way to increase your chance of getting pregnant. Tribal women have been aware of the cyclical nature of fertility and have used it to avoid or achieve pregnancy for centuries. However, it has only been in the last 150 years or so that science has taken an interest in these fertility signs and made them accessible to modern day women.

Not all of the research done on fertility has been helpful. The rhythm method, a mathematical equation developed in the 1930s to calculate fertility, mistakenly assumed that all women have regular cycles all of the time. Menstrual cycles not only vary from woman to woman, but each woman may have cycles of different lengths from month to month--and be perfectly normal. The erroneous logic of the rhythm method has not only resulted in many pregnancies (when used as birth control), but has smeared the reputation of worthwhile fertility awareness methods.

There are several effective fertility awareness systems that will steer you toward the most ideal time to conceive. Three main fertility signs are used in part or totally by these fertility methods. Basal body temperature or BBT, probably the most familiar fertility sign, is the lowest body temperature taken during the day. It identifies ovulation and even pregnancy. When progesterone increases, after ovulation, a woman's temperature rises. Her temperature remains elevated until menstruation and then drops. If she's pregnant, her BBT will remain high for at least 17 days.

Using only BBT to track fertility has its drawbacks. Illness, interrupted sleep, an erratic schedule, or drinking alcohol or eating late at night can give skewed results. It's also important to take your temperature immediately before rising. If this isn't done, your BBT reading can't be used for that day. Sometimes BBT doesn't rise during a woman's menstrual cycle. Other times, her temperature may increase after ovulation. Obviously in these cases, BBT is no help to couples trying to conceive.