There are just four simple rules of the Billings Ovulation Method™ which you can apply in all circumstances and at all stages of your reproductive life:
Avoid intercourse on days of heavy menstrual bleeding
In a short cycle fertility may begin before bleeding has finished, and the bleeding could mask the presence of the mucus which indicates fertility.
You can have intercourse on alternate evenings of your Basic Infertile Pattern (BIP)
You wait until evening so you can be sure there has been no change from your BIP. You need to be upright and moving about for a few hours for the mucus to flow down to where you can become aware of it at the vulva. The reason for alternate evenings is because the day after sex you may feel damp and see a discharge of seminal fluid, and this can mask a change in the mucus pattern. The seminal fluid detected on the day after sex will not contain any live sperm. So even if fertility starts in the hours following intercourse there will be no sperm capable of reaching and fertilising the ovum.
When there is a change from your BIP, wait and see
Waiting without intercourse when there is a change from the BIP allows you to observe whether this change results in a Peak or in a return to the BIP. A change from the BIP indicates potential fertility and the possibility of sperm survival.
If the Peak is observed the Peak Rule can be applied. If the BIP returns you should wait for a further 3 days and then resume using Rule 2 from the fourth evening. This ensures that the hormones have settled back to a low level and the cervix is tightly closed.
You may have intercourse at any time from the beginning of the fourth day after the Peak until your next period
Waiting until the 4th day past the Peak allows time for ovulation to take place and for the life-span of the egg. For the remainder of the cycle you are infertile because the egg is dead and you won't ovulate again until after your next menstruation.
To avoid pregnancy you apply the Early Day Rules and the Peak Rule as above.
To achieve a pregnancy you apply the Early Day Rules so you can clearly identify when you become potentially fertile. Once you notice a change from your BIP, wait without sex until you notice the slippery sensation. The slippery sensation means that you're at your most fertile, and that ovulation is very close. Sex over the days of the slippery sensation and the first day or two past the Peak will give you the best chance of conceiving.
Your Billings Ovulation Method™ tutor can help you interpret your individual patterns of fertility and infertility and to correctly apply these guidelines.