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The science behind ovulation prediction tests

Before you begin the search for a fertility aid or ovulation predictor, it is important to understand the basics. To start, fertility is the ability of a woman to conceive, or get pregnant. For this to happen, a man's sperm has to reach a woman's egg, fertilize the egg and then implant in the uterus. While an egg, or ovum, is viable for only 12 to 24 hours, sperm can last up to three days. Therefore, knowing when the egg is released from the ovary, a process called ovulation, and knowing days ahead of time, can greatly increase a woman's chances of conceiving. The time when she's most fertile, often about five days before ovulation and the day of ovulation, is called the fertile window, and the aim of every fertility device is to predict it.

Predicting the fertility window and ovulation

It used to be thought that ovulation occurred right in the middle of a woman's menstrual cycle. So if she had a 28-day cycle, she would ovulate on day 14 (day 1 being the start of menstrual bleeding). However, research published in the British Medical Journal in 2000 showed that only 10 percent of women with a 28-day cycle followed this pattern. Though there was a higher probability of ovulation occurring around day 14, for women with a regular cycle, their fertile window could occur anywhere from day 6 to day 21. Women with irregular cycles have more variability in when they ovulate, so simply counting days on a calendar will not effectively pinpoint ovulation for those women.

Lots of research has been performed to see what hormonal and physical changes cause and signal the occurrence of ovulation. While the entire menstrual cycle is governed by a complex set of hormones, home ovulation predictor tests have focused on two: luteinizing hormone (LH) and estrone-3-glucuronide (E3G). While LH is present in varying amounts throughout a woman's menstrual cycle, a surge in this hormone is what actually causes her to ovulate within 24 to 48 hours, and the hormone can be detected in her urine.

Detecting the LH surge can give a woman lead time of about one day, but what if she wants to know sooner, thereby maximizing her fertile window? The answer is estrogen. Again, estrogen is present throughout a woman's menstrual cycle, but a surge in estrogen is what triggers the LH to surge. E3G is a metabolite of estrogen and can be detected in a woman's urine. If she knows when her E3G surges, she can predict when she'll ovulate days in advance. All the urine tests currently on the market measure the LH or the LH and E3G to tell a woman when she'll ovulate.

Though they don't directly measure the hormonal changes that signal ovulation, fertility devices do measure the effects these hormones have on other parts of a woman's body. Throughout a woman's cycle, for instance, the hormonal changes cause the amount of salts in her sweat, saliva and vaginal secretions to change. Numerous devices have been developed to detect these changes. They range from sensors for the skin, mouth and vagina to miniature microscopes for examining your saliva -- the saliva, when dried, will crystallize due to the increased salt and take on a characteristic fern-like appearance. This is often called "salivary ferning."

Predicting fertility without tests and gadgets

Ovulation was traditionally predicted by bodily changes that occurred around the fertile window. First, a woman's cervical mucus will thin and become stretchy, akin in consistency to raw egg whites. This is believed to allow the sperm to travel through the reproductive tract more easily. Also, the position of her cervix will change around the time of ovulation. Next, up to 20 percent of women can actually feel the mild discomfort of ovulation. This condition, referred to as mittelschmerz, is often characterized as mild one-sided, lower abdominal pain. Sexual libido is also an indicator. Some studies have shown that the days when a woman is most likely to want to have sex occur during her fertile window.

Lastly, after ovulation has occurred, a woman's temperature rises as her progesterone increases. This increase can vary from 0.4 to 1 degree, but to detect it a woman needs to know her basal body temperature (BBT), or the lowest temperature of the day for her body. This low point occurs when a woman first wakes in the morning and can be affected by the smallest things, like brushing your teeth. As such, very precise thermometers are sold specifically for basal body temperatures. The downside of the BBT is that the rise only occurs after ovulation has occurred. However, if a woman commits to charting her BBT daily, she can use the data from previous cycles and other bodily changes to help her predict when her fertile window is most likely to occur in each menstrual cycle.

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