Here’s when your fertile window opens and closes

Did you know that fertilization can only occur in the 12- to 24-hour period after ovulation? This is because if the egg reaches the uterus after 24 hours and isn’t fertilized, it will dissolve, become part of the uterine lining, and be shed at the next menstrual period.

When are you the most fertile?

Since women typically ovulate just once a month, this might seem unfairly short! Unlike your egg, however, sperm are resilient swimmers – they can remain viable in your body for up to five days. So while you’re more likely to conceive through intercourse nearest the day of ovulation, it’s possible to conceive over a full period of six days. Known as the “fertile window,” this timeframe includes the day of ovulation (the 12 to 24 hour period after an egg is released), and the five days prior.

To find that sweet spot in this limited time opportunity, many couples who are trying to conceive find that tracking their fertile window and the menstrual cycle can greatly improve their timing and chances of getting pregnant from month to month.

Use the Ovia Fertility Score to estimate the fertile window

Ovia provides you with a Fertility Score, a number from 1-10 that indicates how fertile you are on any given day throughout the cycle, with 10 being the highest and 1 the lowest. A Fertility Score of 7 or above means that you are likely fertile at that time. Ovia uses all of the information you provide, like your period dates, cervical fluid consistency, basal body temperature, symptoms, and moods to accurately predict your ovulation and fertile window, and tell how ready your body is to conceive throughout different phases of the cycle. Ovia will send you an alert if any data you have entered causes your fertility score to change for a particular day.

The menstrual cycle

Women are born with all of the eggs they’ll ever have, and beginning with the first menstruation, one egg (or sometimes more – this is how fraternal twins are made!) is released each menstrual cycle at ovulation. Aside from those six days each cycle on which intercourse can lead to pregnancy and the days when you have your period, what happens during the other days of the cycle? Let’s take a closer look into how the menstrual cycle works.

The menstrual cycle is broken down into five phases: menstruation, the proliferative phase, the fertile window, and the early and late luteal stages. Many sources cite 28 days as the length of the menstrual cycle, but in reality it differs from woman to woman, and anything from 21 to 35 days is considered normal.


The menstrual cycle begins with a menstrual period, known as menstruation. Menstruation occurs if you did not conceive in the previous cycle, during which time the uterine lining and unfertilized egg from last cycle are shed and your body begins preparing new eggs for ovulation. Hormone levels tend to be at their lowest during menstruation, as the period is triggered by a drop in progesterone levels. When you near the end of the menstrual phase, your estrogen levels will begin to rise in order to stimulate the eggs developing in follicles in your ovaries.


The proliferative phase (also known as the follicular phase) technically begins on the first day of menstruation, but is typically considered to start once your period is over. The proliferative phase is the time during which the eggs housed in follicles in your ovaries are maturing–with the help of your rising estrogen levels–and competing with one another to become the sole egg available for ovulation. Although the proliferative phase continues up until ovulation, a short phase known as the “ovulatory phase,” the fertile window begins five days before ovulation. For this reason, it’s often easier to think of the proliferative phase ending at the beginning of the fertile window.

Fertile window

 During the first five days of the fertile window, eggs are maturing in follicles in your ovaries thanks to the help of your still-rising estrogen levels. However, as you near the day of ovulation, your levels of Follicle-stimulating hormone (FSH) rise to help the eggs in your ovaries ripen and get ready for release. Shortly after, you’ll have a spike in luteinizing hormone (LH), which causes one ovarian follicle to rupture and your ovary to release an egg. Once the egg is released, there are only about 12-24 hours remaining in which the egg is viable. Progesterone levels are also rising during this time, as it is the hormone most important for supporting a pregnancy.


Early luteal

After the fertile window ends and your egg is either fertilized or not, the luteal phase begins. Although the luteal phase is really only one phase, it’s helpful to think of it in two halves–early and late–because of some differences between the beginning and end. During the early luteal phase, your progesterone levels will rise, as this is the hormone responsible for helping you sustain an early pregnancy. If you conceive in a cycle, your fertilized egg will make its way down the fallopian tube and implant in your uterine wall, about 7-10 days after ovulation, and it is not until this time that the earliest signs of pregnancy may begin to appear. Implantation marks the transition between the early and late luteal phases.

Late luteal

The second half of the luteal phase begins when an egg has either been fertilized and is implanting in your uterine lining, or has been absorbed by your body. If your egg was fertilized and implanted, your progesterone levels will remain elevated during the late luteal phase in order to help support your pregnancy. You will also start to produce hCG when the egg implants, which may cause some early pregnancy symptoms. If you do not conceive in a cycle, your progesterone levels will drop, triggering a period, and a new menstrual cycle.

Read more


  • Murcia-Lora, José María; Esparza-Encina, María Luisa. “The Fertile Window and Biomarkers: A Review and Analysis of Normal Ovulation Cycles.” Persona y Bioética. Vol. 15 Issue 2, p133-148. 16p. Web. July-December 2011.
  • Stanford JB, Dunson DB. “Effects of sexual intercourse patterns in time to pregnancy studies.” American Journal of Epidemiology. 165(9):1088-95. Web. 5/1/2007.
  • Pearlstone AC, Surrey ES. “The temporal relation between the urine LH surge and sonographic evidence of ovulation: determinants and clinical significance.” Obstetrics & Gynecology. 83(2):184-8. Web. Feb-94.
  • Mayo Clinic Staff. “How to get pregnant.” Mayo Clinic. Mayo Clinic, 2/14/2014. Web.
  • “ASRM Patient Fact Sheet: Optimizing Natural Fertility.” ASRM. American Society for Reproductive Medicine, 2012. Web.

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