When Is a Woman Least Fertile?

Fertility depends on a variety of factors. Statistically, your chances of becoming pregnant are highest in your 20s and 30s when you have a higher quantity and quality of eggs. 

Even during these years, your level of fertility fluctuates throughout your menstrual cycle and relies on ovulation. Several factors can impact a woman’s fertility, so it’s important to be in tune with what’s happening in your body to improve your chances of getting—or not getting—pregnant.

You Are Least Fertile During Menstruation

Simply put, your least fertile days are during the menstruation phase of your menstrual cycle. To be more precise, according to the Human Reproduction Journal, the chances of getting pregnant are lowest on Day 4 of your menstrual cycle (2% probability, but not zero!).

In order to become pregnant, an immature egg (or oocyte) has to be fertilized by a single sperm. Ovulation is a process that occurs on approximately days 13 to 15 of a 28-day menstrual cycle, during which your ovary releases an oocyte from a follicle. The oocyte then travels to the fallopian tube and awaits fertilization with sperm.

Once ovulation occurs, the oocyte can be fertilized by sperm. At this point, timing is of the essence. After ovulation, you have about 12–24 hours for fertilization to occur. However, it’s worth noting that sperm can live inside a female body for up to 5 days, so the window for intercourse to result in pregnancy is longer than just a couple of days. 

If the egg isn’t fertilized within 12–24 hours of its release, it begins to disintegrate slowly and will eventually shed along with the uterine lining during menstruation (your period), approximately 11–16 days later.

Your Menstrual Cycle, Day by Day

Your period marks the first day (Day 1) of your cycle. It’s important to note that sperm can survive in the female reproductive system for up to 5 days, so if ovulation occurs less than 5 days after intercourse, it is possible to get pregnant. If you have a 28-day menstrual cycle and ovulate regularly, the below is an outline of potential levels of fertility throughout the menstrual cycle.

• Days 1–7: Menstruation (shedding of the uterine lining) = Very low level of fertility 
• Days 8–9: Post-Menstruation = Low–medium level of fertility 
• Days 10–12:  Pre-Ovulation = Medium level of fertility
• Days 13–15: Ovulation = Very high level of fertility 
• Days 16–18: Post-Ovulation = Medium–high level of fertility 
• Days 19–28: Pre-Menstruation = Low level of fertility  

Signs That You’re Ovulating

Many women do not have a cycle length of exactly 28 days, so tracking your own menstrual cycle can help you understand on which days of your menstrual cycle that you ovulate. . Here are some things to look out for to know if you’re nearing or in the ovulation phase of your cycle:

• Lower basal body temperature (this typically occurs just before ovulation begins): It averages between 97°F (36.1°C) and 97.5°F (36.4°C) before ovulation, according to Healthlink BC
• Higher basal body temperature (occurs approximately 24 hours after the egg is released and lasts for many days): It ranges between 97.6°F (36.4°C) and 98.6°F (37°C) after ovulation, according to Healthlink BC
• Cervical mucus that is wet and stretchy with an egg white texture: You may be at your day of ovulation
• Softening of the cervix: Sometimes known as having a short, high, open, and wet cervix (SHOW)  
• Subtle cramping 
• Light spotting
• Increased libido (sex drive)  

The Fertile Window: Take Note If You’re Trying to Get Pregnant

For those trying to get pregnant knowing when you might be ovulating is important. Your most fertile days ("the fertile window") are the 5 days leading up to and including ovulation. The fertile window is when you’re most likely to conceive and have the greatest chance of getting pregnant. 

So, having sex during your fertile window gives you the best chance of ensuring the sperm is present when the egg is released so that you can successfully beome pregnant.

Age and Fertility

Of course, fertility can be impacted by several factors. Lifestyle andhealth conditions can impact fertility; however, age is one of the biggest natural reasons fertility can be decreased.

Your odds of getting pregnant are highest in your 20s and 30s, as this is when women are technically the most fertile and have the highest quantity of qualityeggs. After the age of 35, the chances of becoming pregnant begin to decline And by the time a woman turns 40, her chances of getting pregnant decrease to 10%, per menstrual cycle..

Using Birth Control to Prevent Pregnancy

There are many ways to mitigate the chances of becoming pregnant if you do not want to be. There are over-the-counter and prescription contraceptives available as well as natural methods.

Some common forms of birth control include:

• The birth control pill
• Intrauterine Device (IUD)
• Female or male condom
• The fertility awareness method

How the Birth Control Pill Works

"The pill” is a form of oral contraception that uses synthetic hormones to prevent pregnancy. According to the Center for Disease Control (CDC), the pill is 99.7% effective with perfect use. With typical use, however the failure rate decreases to ~93%.

The pill releases synthetic hormones into the female body that prevent ovulation and thin the uterine lining.. So, even if an oocyte was released, it would have nowhere to implant, making pregnancy much more difficult. Many birth control pills are available on the market, each with different hormones, doses, and varying side effects. It’s important to talk to your doctor and do your research before choosing a birth control pill.

How IUDs Work

An IUD is a small contraceptive device that’s inserted into the uterus. The device stays in the female body from 3 to 10 years before they need to be replaced.

IUDs can be made from copper or a flexible plastic and work by preventing the sperm from fertilizing the egg. There are non-hormonal and hormonal IUDs. Hormonal IUDs also release a hormone that prevents ovulation, implantation, and/or fertilization, whereas non-hormonal IUDs create a uterine environment that prevents implantation and fertilization.

Female and Male Condoms

Condoms are a commonly used form of contraception that not only prevent pregnancy but can protect against sexually transmitted infections (STIs) as well. They do this by forming a physical barrier that prevents sperm and sperm fluid (seminal fluid) from entering the vaginal canal and fertilizing an egg.

With perfect use, condoms are said to be 98% effective at protecting against unwanted pregnancy. More often than not, however, human error leads to this statistic lowering to 85%. 

Fertility Awareness Method 

You can use your fertile window as a way to manage the chances of becoming pregnant. Your most fertile days ("the fertile window") are the 5 days leading up to and including the day of ovulation.

Using a calendar, you can track your menstrual cycle over the course of several months. Based on the average length of your cycle, you can somewhat accurately predict your fertile window and plan when to have (or abstain) from unprotected sex to increase (or decrease) your chances of getting pregnant. This is sometimes known as the “calendar method” or “rhythm method.” 

Remember that there are several factors that affect your cycle from month to month. According to the Centers for Disease Control and Prevention (CDC), failure rates for this method range from 2%–23%. 

It’s still best to avoid having unprotected sex if you don’t want to get pregnant. Contraception is the next best thing to avoiding unwanted pregnancy, second to abstinence.

Other Factors That Cause Infertility in Women

Ovulation is a intricate process. Your cycle can be impacted by factors like stress, exhaustion, or lifestyle choices like nutrition, alcohol use and smoking. 

However, there can also be underlying health reasons that lead to infertility, as well. Irregular periods or absent periods could be an indicator that ovulation is not occurring. If you experience this you should always visit a doctor.

Medically reviewed by Dr. Chimsom T. Oleka, M.D, Written by Jane Flanagan — Updated on November 1, 2021.

We hope you found this post informative — but remember: we’re not doctors and this post is not medical advice! While all posts are fact-checked and well researched, we always recommend you chat with your doctor about any questions or concerns you might have regarding a medical condition. We’re here to support and educate, but never with the aim of disregarding professional medical advice you’ve been given. Phew, now that that’s out of the way, you can go on living unapologetically free.