How to identify your fertile days

Fertile days are the time when you can get pregnant (given that you don’t use hormonal contraception, such as the pill or a hormonal IUD). The fertile days begin five days before ovulation (because sperm can survive in your body for a maximum of five days) and last up to one day after ovulation. However, only 6 percent of conceptions can be attributed to fertilization by sperm three or more days old.

Ovulation takes place approximately in the middle of your cycle. For example, if your cycle is 28-29 days long (first day of menstruation = day one of your cycle), it is likely that your ovulation will take place between the 13th and 15th day or your cycle. Your fertile days would, therefore, not begin until the 9th day of your cycle at the earliest.

The most fertile days in your cycle are the two days before ovulation and the day on which ovulation takes place. Since the oocyte can only be fertilized for 12-24 hours, the probability of pregnancy  after ovulation quickly drops to zero.

Accurately determine the most fertile days of your cycle

If you would like to get pregnant fast, you will want to know the fertile days as precisely as possible. The five best ways of doing so are:

  • An ovulation calendar or online fertility tracker
  • Ovulation tests measuring the luteinizing hormone (LH) in your morning urine
  • A Billings test (analyzing your vaginal discharge)
  • Wearables
  • Ultrasound monitoring

If you have irregular cycles or very long cycles (35+ days), you should get your hormones checked. You may have polycystic ovary syndrome (PCOS).

Identify your fertile days #1: ovulation calendar

An ovulation calendar helps you get to know and interpret your body better. Ovulation calendars are available on paper or as an app. The advantage of apps is that you’ll be notified when your fertile days are about to start and when to expect your period. Typically, a fertility app will ask you to enter the days of your period and the strength of bleeding and the days when you have vaginal discharge, as well as symptoms such as abdominal pain, headaches, or depressive moods. Apps that take actual measurements into account (such as body temperature) are much more accurate.

Apps can help you get pregnant faster, but they are not suitable as a reliable method of contraception, especially if a pregnancy is out of the question for you. If you have PCOS, apps are usually not suitable for determining your fertile days.

Identify your fertile days #2: ovulation test with urine

Ovulation tests with urine measure the luteinizing hormone (LH). Ovulation is triggered by a sharp increase in LH. You can expect to ovulate about 24-48 hours after the LH increase. It is the most fertile time in your cycle.

Urine tests must be carried out once a day at the same time of day for five to seven days (preferably in the morning). I personally had a good experience with ovulations tests and conceived my second daughter using them. Nevertheless, I am well aware that they are not always clear, and I, too, struggled to interpret them correctly.

The cycle day on which you should start testing depends on your cycle length. As a rule, you can subtract 17-18 days from your usual cycle length to come up with the day to start. For example, if your cycle usually lasts 28 days, you should start testing on day 10 or 11. Ovulation tests will show two lines once you apply urine on them: a control line and a test line. When the test line appears just as dark as the control line, the LH surge has taken place, and ovulation is imminent.

Unfortunately, the increase in LH does not guarantee that ovulation is taking place. Occasionally, there is an LH surge without an ovulation. On the other hand, ovulation sometimes occurs without a detectable LH surge. Therefore, ovulations tests are not a good means of contraception.

Identify your fertile days #3: cervical mucus method (Billings Ovulation Method)

The cervical mucus method is a form of natural family planning and is used by women for both contraception and conceiving. The mucus method goes back to the Australian scientist John Billings, who researched the nature of the cervical mucus. Billings found that the otherwise somewhat sticky mucus changes just before ovulation: it becomes clear and wet.

If you do not use hormonal contraception, you will notice an increase in vaginal discharge about seven to 12 days after your last period. Some women compare the consistency of the mucus to an egg-white. You’ll be able to notice this consistency for about five days. Then, the discharge will become stickier, marking the end of your fertile days. 

Some women, especially those with PCOS, have several patches of fertile-looking cervical mucus throughout their cycle. If this is your situation, predicting ovulation by tracking cervical mucus might not work well for you.

Identify your fertile days #4: Wearables

Wearables are devices you wear on or in your body. They are equipped with sensors that collect signals from your body, send them to an app on your phone and analyze them. Examples of wearables are bracelets, skin patches or vaginal sensors.

Wearables work the better the longer you wear them. Because most women would like to get pregnant fast – i.e., during their first three cycles of trying – it may not be worth buying a rather expensive wearable. If you have been trying to get pregnant for a longer time, however, they can be a good support tool to accurately determine your fertile days. Some women find that they ovulate three to four days later or sooner than they thought.

If you have PCOS, make sure that your wearable of choice is suitable for this condition.

Identify your fertile days #5: ultrasound

The safest way to predict ovulation (and the only way to prove you’re ovulating) is with an ultrasound. Your gynecologist can see how many follicles are developing and what stage they are at. The size of the biggest follicle can be used to predict when ovulation will take place.

Ovulation monitoring via ultrasound is routinely done during in vitro fertilization (IVF) to make sure hormonal stimulation is working and enough follicles are growing.

If you’re trying to get pregnant and you suspect you aren’t ovulating, then an ultrasound exam, as well as ovulations tests, can give you clarity. An ultrasound examination is also important if PCOS is suspected. 


Wilcox AJ et al: Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517-1521.

Bigelow JL et al: Mucus observations in the fertile window: a better predictor of conception than timing of intercourse. Hum Reprod. 2004;19(4):889-892.