Even with regular periods, there can be some variation in the fertile window from month to month. There are several ways that people track ovulation and time intercourse when trying to get pregnant:
Ovulation Predictor Kits
Home ovulation predictor kits (OPKs) typically test urine for the rise in Luteinizing Hormone (LH) that occurs just before ovulation. If you are using OPKs, we recommend starting to test daily several days before you think you will ovulate — this is usually around day ten of your cycle if you have 28-day cycles. If your OPK goes from negative (not close to ovulation) to positive (LH surge detected), you will most likely ovulate in the next day or so; sex on the day of the positive OPK will have the highest chance of pregnancy. We typically recommend digital OPKs (such as Clearblue Easy) that give a yes/no answer since they can be easier to understand.
Importantly, there may be false negatives (i.e. you are actually close to ovulation but the test is negative), for example, if your sample is too dilute for the test to pick up a high LH level. There may also be false positives (i.e. you are not ovulating but the test if positive), for example, if you have polycystic ovary syndrome (PCOS).
Calendar Method
Fertility tracking using a calendar or a smart phone app can help to determine the length of your cycles and the most likely time of ovulation. Ovulation usually occurs 14 days before the start of the next period — for example, this would occur around cycle day 14 if you have a 28-day cycle, or around day 16 if you have a 30-day cycle. Becoming familiar with your cycles may help you predict when you should have sex to optimize chances of conception.
Basal Body Temperature
Basal body temperature involves monitoring your temperature first thing in the morning. Your temperature will rise at the time of ovulation, sometimes by only a fraction of a degree. This method can help you understand and predict your cycles, but since the most effective time to have sex is the day or two before ovulation, tracking your temperature will not give you an indication of when to have sex this cycle.
Cervical Mucus
Your cervical mucus may vary throughout the month, and the quality and amount of the mucus may help you determine when you are close to ovulation. Larger volume, slippery, clear "egg white" mucus is generally noted when you are approaching ovulation.
The process of getting pregnant may be stressful, and some patients find that testing using these various tracking methods adds an additional level of anxiety. While the information from these tests may be helpful to time intercourse at the highest yield time, none of the methods mentioned above have been shown to actually increase pregnancy rates in people having regular intercourse. If tracking your cycles is becoming stressful, it is important to remember that these methods are not necessary for conceiving and having sex even without tracking is still effective if you have predictable cycles!