When Is a Woman Least Fertile?


Fertility depends on a variety of factors, some natural and some not. Generally, your odds of becoming pregnant are highest in your 20s and 30s when you’re technically the most fertile and have higher quality eggs. 

Even during these years, your level of fertility fluctuates throughout your menstrual cycle and depends on when you’re ovulating. 

There are other factors that affect a woman’s fertility like lifestyle and medical conditions, 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 is during menstruation. 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).

In order to become pregnant, an egg (or ovum) has to be fertilized by a single sperm. Ovulation is a monthly event when your ovaries release an egg into your fallopian tube. It usually occurs around 14 days into your cycle and lasts an average of 12 to 48. After that, the egg begins to disintegrate. Ovulation represents your most fertile days since an egg needs to be present in order for fertilization to take place.

During menstruation, the disintegrated egg sheds along with the uterine lining in what’s commonly referred to as a period. 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 five days, so if ovulation occurs less than five days after intercourse, it is still possible to get pregnant.

Day 1-7:

Menstruation (shedding of the uterine lining) = Very low level of fertility 

Days 8-9: 

Post-Menstruation = Low/medium level of fertility 

Days 10-13: 

Pre-Ovulation = Medium level of fertility

Days 14-16: 

Ovulation = Very high level of fertility 

Days 17-18:

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 try not to think about fertility in terms of Day X of your cycle. 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).
  • Higher basal body temperature (occurs approximately 24 hours after the egg is released and lasts for many days).
  • Cervical mucus that is wet and stretchy with an egg white texture. 
  • Softening of the cervix. Sometimes known as having a short, high, open and wet cervix (SHOW).  
  • Subtle cramping. 
  • Light spotting.
  • Increased libido (sex drive).  

Age and Fertility

Of course, fertility is impacted by many other factors. Lifestyle and other health conditions can have an effect on fertility; however, age is one of the biggest natural factors affecting fertility.

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 quality eggs. After the age of 37, the odds of becoming pregnant are greatly reduced. And by the time a woman turns 40, her chances to get pregnant decrease to 10%, on average.

Using Birth Control to Prevent Pregnancy

There are many ways to mitigate the chances of becoming pregnant. 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 Centre for Disease Control (CDC), the pill is 99.7% effective with perfect use.

The pill releases synthetic hormones into the female body that prevents ovulation. And stops the uterine wall from thickening. So, even if an ovum was released, it would have nowhere to implant, making pregnancy virtually impossible. 

There are many birth control pills 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 Intrauterine Devices (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 are often made of copper or plastic and work by physically 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. 

Female and Male Condoms

Condoms are a commonly used form of contraception that not only prevents pregnancy but can protect against sexually transmitted infections (STIs) as well. They do this by forming a physical barrier that prevents sperm 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 stat lowering to 85%. 

The Fertility Awareness Method of Birth Control

You can use your fertile window as a way to manage the changes of becoming pregnant. Your most fertile days ("the fertile window") are the 3 days leading up to and including 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 things that affect your cycle from month to month so this method is an imperfect science. According to The Centre for Disease Control (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 delicate process and your fertile window can be a moving target. Your cycle can be impacted by factors like stress, exhaustion, or lifestyle choices like nutrition and smoking. 

However, there can also be underlying health reasons that lead to infertility. Irregular periods or absent periods could be an indicator that you aren’t ovulating. If you experience these symptoms you should always visit a doctor.