The fertile window is the handful of days during a woman’s menstrual cycle when it is possible to conceive. In order to conceive, sex has to take place within the fertile window. (If this is the first time you are hearing this critical information, you are not alone. Sex ed classes typically teach that sex at any time can lead to pregnancy). 

The fertile window exists because egg cells do not last forever after ovulation. Once an egg cell is released from the ovary, that egg cell lasts only about twelve hours. At the most, if your egg cells last exceptionally long and you ovulate two eggs, you are fertile for up to 48 hours.  That is a rare exception.

Fortunately, sperm cells can last for up to five days in a supportive vagina/uterus which greatly extends the opportunities for sex that leads to conception. 

There are many ways of detecting the fertile window. 

  1. Testing leutenizing hormone (LH) levels. The surge of LH triggers ovulation. LH levels can be detected in urine, by using LH strips, ovulation predictor kits or electronic fertility monitors.
  2. Basal body temperature monitoring. After ovulation occurs, body temperature rises in response to higher levels of the hormone progesterone. This change occurs after ovulation, so it cannot predict ovulation. However, if you have regular cycles, you can track your temperature change over a few cycles and then know pretty well when you typically ovulate. 
  3. Detecting cervical fluid changes. To help with conception, fertile quality cervical fluid, which looks and feels like egg white, is produced around the time of ovulation. You may see it on your underwear, on toilet paper, or on your finger if you do an internal exam. You know it is fertile quality if it stretches between your fingers. 

The are other methods of detecting ovulation, but these are the three most reliable ones that can be used at home.

Trying to time sex to correspond to the fertile window is called ‘timed intercourse’ in the scientific literature. A large review of timed intercourse had these findings:

  • The overall quality of the information we have about whether or not timed intercourse is helpful is poor
  • As best the authours could determine given the quality of the evidence, timed intercourse does improve the chance of becoming pregnant

Guidelines for doctors assessing infertility in the US and the UK advise against recommending timed intercourse because it can be stressful. I find this viewpoint to be paternalistic. Some couples might find it stressful but others will feel empowered by having the information and increasing their knowledge of their cycle patterns. Whether or not to do it should be the couple’s decision.

As a naturopathic doctor with a special interest in this area, information from LH levels, basal body temperature charts and pattern of cervical fluid can provide me with useful information that can impact the treatment plan I recommend. In most cases, a little extra stress is a good trade-off for that information.

Cochrane Database Syst Rev. 2015 Mar 17;(3):CD011345. doi: 10.1002/14651858.CD011345.pub2. Timed intercourse for couples trying to conceive. Manders M1, McLindon L, Schulze B, Beckmann MM, Kremer JA, Farquhar C.

Fertil Steril. 2012 Aug;98(2):302-7. Epub 2012 Jun 13. Diagnostic evaluation of the infertile female: a committee opinion. Practice Committee of American Society for Reproductive Medicine.

Fertility: assessment and treatment for people with fertility problems. National Collaborating Centre for Women’s and Childrens Health. Commissioned by the National Institute for Clinical Excellence. Feb 2004. RCOG Press.