Lets use an example to help show the range of fertility someone with a regular cycle might have. Just to refresh your memory - when counting the menstrual/fertility cycle - the first day of the menstrual period is Day 1.
In this example, our subject has been charting her cycle by marking the first day of bleeding each cycle for many years and counting the days in between. But she does not know how to identify fertility or ovulation. From this information she knows the shortest cycle she has ever had was 25 days long and her longest cycle on record is 32 days. These long and short cycles do not occur often and are not the norm for her. Her period normally starts between Day 27 and Day 29. When determining her range of fertility she will take all this information into consideration, helping her to identify when she may be fertile. Once she knows when she may be fertile she can gauge her risk of pregnancy.
Pull out your calendar, if you can make a copy of it even better, then you can write on it as we go.
Now she will use the information she has collected over the years to make an educated guess which will be based upon her personal history. She knows the first day of her last period was March 1st. To figure her range of fertility she would use a calendar. Starting with Day 1 (the first day of her period, March 1st) she would count forward 25 days (her shortest cycle on record); which is March 25th - now she would count back 14 days (the fourteen days between ovulation and menstruation). This would be March 12th. Based on her past history (from charting her cycles) this would be the earliest she might ovulate, but because she rarely menstruates as early as Day 25, chances are she would not ovulate this early, but it is possible and should be taken into consideration.
After determining the earliest possible date for ovulation this cycle (March 12), she would need to take into account the life-span of the sperm, which would be no more than 7 days and could be less. She would count back 7 more days would put her at March 5th. This means that any unprotected sex after March 4th would put her at risk for pregnancy but the risk would be low because she does not usually ovulate this early.
Normally her average short cycle is 27 days. So using the same method as above she can determine that any unprotected sex on or after March 9th would put her at high risk for pregnancy. {March 9th is arrived at by starting with March 1 as Day 1; counting forward 27 days - March 27th; then counting back 14 days (March 14); then counting back 7 more days to protect from sperm - making it March 9th.}
Now to determine the upper range of her fertile time for any given cycle we would use both the 29 day norm for her as well as the 32 day record for longest cycle. To determine her most fertile time, from the 29th Day of her cycle which in this example would be March 29th she would count back 14 days making it March 16th, now add two days to that, the length of life for the ovum (it can be fertilized for 24-36 hours after being released from the ovary) Making it March 18th. So her most fertile time is between March 9th through March 18th. Any exposure to sperm at this time is likely to result in pregnancy, assuming she ovulates on schedule.
And finally figuring the outer range of her fertility for a given cycle would be to take the 32 Day record for her longest cycle. Starting with Day 1 (March 1st) counting forward 32 days making it April 1st; counting back 14 days = March 19th. This should be the latest ovulation might occur. Again, for her to ovulate this late would be unusual for her, but not impossible. Adding two days to March 19th makes it March 21st giving the egg time to expire. This is the latest she would be fertile, again, this is assuming she doesn't set a new record for late ovulation this cycle. There is always the element of the unknown.
Now she has determined her personal range of fertility for any given cycle as long as her earliest and latest records do not change. Her most fertile time is between Day 9 (March 9th) and Day 18 (March 18th) of her cycle, any unprotected sex during this time is likely to result in pregnancy and would be considered moderate to high risk. Because not all sperm can live for 7 days - if unprotected sex occurred on the 9th or 10th Day of her cycle, her risks of pregnancy would be slightly less than if the exposure to semen occurred closer to ovulation.
From Day 5 (March 5th) to Day 8 (March 8th), she may be fertile, but probably is not. And from Day 19 (March 19th) to Day 21 (March 21) she may be fertile, but probably is not. So her risk for pregnancy during the dates of March 5th-8th and March 19-21 is low to moderate. Any exposure to sperm occurring between March 9th thru March 18th carries a much higher risk of pregnancy. And finally, any exposure to sperm before March 5th is not likely to result in pregnancy. And any exposure to sperm after March 21st is unlikely to result in pregnancy since ovulation should be finished and she should be non-fertile for the remainder of the cycle. These would be low risk times, and a wait-and-see approach is appropriate.