Predicting Your Most Fertile Periods
Understanding how the menstrual cycle functions - and what happens during ovulation - is key to the successful prediction of ovulation. During your menstrual cycle, there are far more infertile days than fertile days. The fertile days are days when there is a possibility for conceiving. However, within your fertile period - the window of opportunity for conception - there are days of "peak fertility". Identifying these peak times which can be done most effectively through the use of a monitor, and planning accordingly, can help maximize your chances of becoming pregnant.
Counting the days of your cycle can tell you how long your cycles are and whether or not they are regular, but the most reliable way to monitor changes in your fertility is by paying attention to clues from your body. Changes in your cervical mucus, basal body temperature and the position of your cervix can help you follow your cycle from start to finish. Some women chart all of these changes carefully to either increase or reduce their chances of becoming pregnant. Some women follow their cycles to know when to expect their periods, or to feel more in tune with their bodies.
Cervical Mucus
Mucus from your cervix changes throughout your cycle in response to hormonal changes. It’s a good marker of where you are in your cycle and checking your cervical mucus is quick, easy and reliable. Some women produce more mucus than others and may be able to see the mucus in their underwear. Others may need to feel the entrance of their vagina for it – use your fingertips or toilet paper. You can check your mucus at any time during the day.
At the beginning of your cycle, when oestrogen levels are low, you won’t have much mucus at all. This dryness means you are relatively infertile. This is usually just after your period, but if you have a short cycle, it may be at the same time as your period and therefore difficult to monitor.
A few days later, depending on the length of your cycle, your mucus will begin to increase. It may be sticky, white, milky or cloudy. This is a signal that you are entering your fertile window and nearing ovulation. If you are not using contraception and do not want to get pregnant, you should avoid sex from this point until a few days after you stop seeing egg-white mucus (see below). If you do want to get pregnant, this mucus marks the beginning of your fertile time.
Just before ovulation, a woman’s mucus becomes slippery, stretchy and clear – like raw egg-white. This egg-white mucus helps sperm move from the vagina through the cervix and into the uterus in search of an egg. This is the most fertile time in a woman’s cycle.
After ovulation, the mucus changes from the egg-white type back to a thicker, cloudier mucus. You may still be fertile if you see this mucus, but only for two or three days at most. Four days after your last egg-white mucus, you will be in the infertile phase of your cycle.
Your cervix still produces mucus after this point but under the influence of progesterone the mucus is thick, sticky and acidic. It stays around the cervix to block the sperm and many women feel a dryness compared to earlier in their cycles. Some women have no noticeable mucus during this stage, while others notice a thick or white mucus.
Checking your mucus is one of the easiest and most accurate ways to follow your cycle, but changes in mucus can be difficult to detect if you have a vaginal infection or use spermicide. Antihistamines can also cause your cervix to produce less mucus, making it difficult to check.
Changes in your cervix
You can also note the different stages of your cycle by feeling your cervix. To feel the changes in your cervix, insert one or two fingers deep into your vagina until you can touch what feels like the tip of your nose. This is your cervix. You may be able to feel an indent or dimple in your cervix. This is the opening to your uterus – called the os.
At the beginning of your cycle, your cervix should feel firm and low. The os will be closed. But as you approach ovulation, the position and feel of your cervix changes. It may feel softer and higher in your vagina and the os will feel more open. This means you are in your most fertile phase.
After ovulation, the cervix becomes firm and closed again and moves lower in the vagina. When the cervix has been like this for 3 days, you are no longer in the fertile phase.
It may take a few cycles to recognise the changes in your cervix and get to know what is normal for you. If you are trying to get pregnant or avoid getting pregnant, you may want to check your cervix at about the same time every day. This is the most reliable way to track changes.
Basal body temperature
Basal body temperature is your waking temperature first thing in the morning – before getting out of bed or doing anything else. You can’t feel these slight changes in your body temperature but a basal thermometer can detect them. You can take your temperature orally, vaginally or rectally as long as you use the same method every day. Before ovulation your temperature may vary from day to day by about 0.1 degree Celsius. Just after ovulation, however, your temperature rises by at least 0.2 degrees Celsius and remains high until your next period. This increase in temperature is caused by the progesterone released from the follicle after ovulation.
It is important to note that the rise in your basal body temperature doesn’t tell you when you are fertile or about to ovulate. It tells you when you already have ovulated. Women are most fertile the few days before their peak temperature, and are least fertile once their temperature has remained high for three days.
Tracking your temperature is the most demanding method of charting your cycle. You must take your temperature at the same time every morning, as soon as you wake up and after at least three hours of sleep. Going to the bathroom, having a cup of tea or moving around too much can all change your temperature and make it more difficult to see a regular pattern. Other things can also affect your temperature, such as illness or infection, alcohol and some prescription and recreational drugs.
If you are keeping track of your basal body temperature, it may help to keep daily notes of the time you take your temperature and any thing else that may affect the reading. Sleeping in for an hour longer than normal, for example, could result in a rise in temperature that looks like ovulation, but isn’t. Download our free BBT chart here
Other signs of your cycle
You may notice other changes that mark the different stages of your cycle. Some women regularly experience abdominal pain (slight or severe), swollen breasts, lower back pain and mood changes (both good and bad). These types of signs are not very reliable in pinpointing your fertile or infertile phases, but they may help support your other observations.
Timing & Fertility
A woman's fertile period during her menstrual cycle, on average, lasts about 9 days. The most fertile period consists of the few days before ovulation. 
There are a number of reasons why this is the most fertile time. First of all, the amount of cervical fluid increases - and the consistency of the mucus changes substantially (see above). The purpose of this change is to create a healthy medium for the sperm to survive and travel in. Given a fertile environment, sperm can live several days. The egg, on the other hand, can live only 24 hours - so timing is important. At the moment of ovulation, the body temperature begins to rise to create a warmer, more hospitable environment for a fertilized egg. Also, the cervix will start to rise, soften, and begin to open up.
Hormones and Ovulation: The Release of the Egg
During ovulation, a number of different hormonal changes take place. Early in the menstrual cycle, a hormone called Follicle Stimulating Hormone - or FSH - enables your ovaries to nurture eggs. Within the ovaries, follicles house each individual developing egg. The follicles that hold the eggs will secrete estrogen. As the menstrual cycle progresses, the follicle containing the developing egg moves toward the surface of the ovary. Immediately before ovulation, the follicle begins secreting estrogen and progesterone. Estrogen helps the uterine lining thicken and expand, and increases blood flow to the uterus. Progesterone causes the glands of the uterine lining to form secretions that help nourish a fertilized egg.
By definition, ovulation is the process of an ovary releasing an egg from the follicle - permitting the egg to float down the fallopian tubes. On average, the first phase of the menstrual cycle last two weeks. Directly prior to ovulation, another hormonal change takes place - the LH Surge. LH (or Luteinizing Hormone) is the hormone that actually facilitates ovulation: it causes the egg to separate from the ovarian surface. Ovulation predictor tests function by detecting this LH Surge, thus alerting the woman that ovulation is about to take place.
Once released from the ovaries (post-ovulation), the egg can survive for about 24 hours. This means that sperm (which can live several days) must be present to fertilize the egg. Following ovulation, the egg enters the fallopian tubes and continues toward the womb. Typically, conception - the uniting of the egg and the sperm - will take place in the fallopian tube, and then the fertilized egg will continue its passage to the uterus and implant in the uterine wall. For pregnancy to take place, fertilization of the egg must be followed by a successful implantation.
Following ovulation, the luteal phase begins, marked by an increase of the hormone progesterone, which strengthens the uterine lining (endometrium) and causes the body temperature to increase, facilitates changes in the cervical fluid, and alters the position of the cervix.
Within 24 hours, if the egg has not been fertilized, it will simply disintegrate after reaching the uterus. Without fertilization - and implantation - the levels of others hormones will ultimately drop during the luteal phase, causing the lining of the uterus to break down and shed - referred to as menstruation, or a woman's "period".
Implantation and Pregnancy
Fertilization occurs when a sperm penetrates the egg - and this typically takes place in one of the fallopian tubes. The fertilized egg then travels to uterus and implants in the uterine lining. At this point, the egg - and developing placenta - begin to release hCG (Human Chorionic Gonadotropin). The presence of hCG will help facilitate the continued production of progesterone - essential for a fertile, hospitable environment for the implanted egg. The hormone hCG will increase - and show up in a woman's blood and urine - making hCG a prime marker for pregnancy detection. Home pregnancy tests function by detecting hCG in a woman's urine.
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Credit: Image modified from Human Biology, 2nd Ed. by Cecile Starr & Beverly McMillan (1997, Wadsworth)