Maybe you are trying to make a baby and you want to know when your attempts are least likely to be rewarded. Or perhaps you would prefer to avoid creating a person and you are curious about how best to go about that. The truth is that you can learn quite a bit about your fertility by paying attention to clues your body gives you about where you are in your cycle.
How your cycle works
The typical menstrual cycle, which runs an average of 28 days and often ranges from 25 to 45 days, starts with your period. Your period, also known as menstrual bleeding, is your body shedding your uterine lining. Every month this lining builds up so that an embryo has a cozy place to grow in case you do get pregnant. You bleed for a few days, sometimes up to a week, as your ovaries prepare to release an egg in a process called ovulation. After ovulation, which generally occurs about midway through your cycle, your body prepares to sustain a pregnancy or for your period to start again.
Understanding fertility clues
You can become pregnant when viable sperm meets a viable egg, but understanding your fertility is more complex than that. For most people’s bodies there is a rhythm to the hormone levels that control ovulation and menstrual bleeding. In order to figure out how it works in your own body, you can track several clues over the course of your cycle.
The first and simplest clue to track is your period. You know when you start bleeding and you can pay attention to when it ends. The second clue that can help you understand your fertility is your basal body temperature—the temperature of your body when you first wake up in the morning, but before you have gotten out of bed. Most people have a temperature lower than 98˚F before ovulation, and higher than 98˚F after ovulation, but the key is that your temperature should increase at least half a degree after your body has released an egg. The third clue that can give a hint about where you are in your cycle is cervical fluid, which is released from your cervix. You can check your cervical fluid by reaching inside your vagina or by inspecting toilet paper after using the restroom. The quality of your cervical fluid changes throughout your cycle and can be sticky, slippery, thick, wet, dry, or even absent. 
When am I least fertile?
If the egg does not meet a sperm for fertilization, then it dies within 24 hours.  But sperm can live inside your body for four or five days, if your cervical fluid is slippery and wet. Thus your most fertile time is the four or five days before your ovaries release an egg and the day or two after. In most people the beginning of this fertile window is marked by an increase in slippery, wet cervical fluid. Some people also feel physical sensations—such as a pinch or a cramp—when the egg is released.
You are the least fertile when you are past the point in your cycle when your body has already released an egg. So if cervical fluid has dried up and your basal body temperature has increased, you likely have already ovulated. From two days after ovulation up to your period starting again, it is very unlikely that you will get pregnant, even if you do have unprotected sex.
How reliable are these fertility clues?
Fertility clues such as basal body temperature and checking cervical fluid can expertly predict your most and least fertile times, but only if you are diligent about checking them. If you do not check basal body temperature every morning when you wake up or fail to pay attention to your cervical fluid, you could miss fertile signs. And thought many people with a 28-day cycle will ovulate around day 12, 13, or 14, that is not always the case. Some people ovulate later or earlier in their cycles and some people have longer cycles, which can shift the fertile window. If you want to avoid pregnancy, the best thing to do is to use a back up method of birth control during any time you might be fertile or throughout your cycle.