Conception 101: How does it work?

Conceiving a baby isn’t simply a story of when sperm meets egg. There are a host of factors that must harmonize to execute this miracle of biological timing!

Functionally, a pregnancy only occurs when a healthy egg is fertilized by a healthy sperm cell. While certain phases of your menstrual cycle can make this meeting easy (hello, ovulation), other phases make it near impossible—though not entirely.

Before we dive into the exceptions, let’s outline the timing behind most instances of conception.

Ovulation Cycles: When Pregnancy Is Most Likely to Occur

The single most important development in the making of a baby is a successful ovulation cycle. During ovulation, an egg is released from one of the ovaries to wait in the fallopian tubes. Ovulation is the body’s only window of opportunity for a sperm cell to successfully bond with the egg.

When a sperm fertilizes an egg successfully in the fallopian tubes, the resulting zygote will travel down to the uterus, where it implants in the uterine lining and begins the process of becoming a baby.

Importantly, a sperm cell and an egg must both be alive to form a fertilized egg. The length of your average reproductive lifespan is as follows:

• Eggs, or ova, stay alive for 12 to 24 hours after being dispensed into the fallopian tubes
• Sperm cells can stay alive for up to 5 days inside a woman’s reproductive organs

Though ovulation only occurs on one day of your menstrual cycle, its impact lasts for longer than 24 hours. Because sperm can live up to 5 days, your conception window lasts from 5 days before ovulation to one day afterward. Technically, it is physically impossible to get pregnant outside of that 6-day window.

Even so, the timing of this window differs from woman to woman—and sometimes between menstrual cycles in the same individual. With that, let’s break down how individual menstrual cycles can impact fertility.

Menstrual Cycles and Fertility

Every woman who menstruates experiences four phases in their monthly reproductive cycle. While the length of cycles and individual phases can vary (and, in many cases, overlap), each phase is generally associated with higher or lower probabilities of becoming pregnant:

1. Menstruation – Your period marks the first day of your reproductive cycle, lasting anywhere from 3 to 7 days. During this stage, your uterus sheds its lining, along with any unfertilized egg released in your previous cycle. Fertility is very low during menstruation, due to this shedding of the egg. However, there is a marginal chance an unfertilized egg may still be fertilized in this phase (more on that later).
2. Follicular – Technically, your follicular stage encompasses the first day of menstruation until ovulation. This is the time when your body produces follicle-stimulating hormone (FSH), which will later prompt your ovaries to release an egg.However, when most people refer to the “follicular phase,” they’re referring to the week-long period between when your period ends and ovulation occurs. Your fertility is relatively low at the beginning of your follicular phase, since sperm will not be able to survive long enough to reach a newly-ovulated egg.
3. Ovulation – About two weeks into your cycle, your ovaries release an egg to travel down your fallopian tubes. This is your highest fertility window. For 92% of women, the ovulation period occurs between days 10 to 14 of their cycle. And while ovulation is a one-day event, peak fertility can encompass the 5 days leading up to ovulation (because sperm can survive for up to 5 days, and can still fertilize an egg).
4. Luteal – Post-ovulation, your body tries to create a welcoming home for any potentially fertilized egg by switching into luteal mode. The corpus luteum, a sheath that once encased your mature egg, emits progesterone to prepare the uterine lining for implantation. However, this lining will be shed during your period if your egg remains unfertilized. Fertility here is low, since most ovulated eggs die at the start of this stage.

A woman’s reproductive cycle can last anywhere from 21 to 40 days, though the average length for most women is 28 days.

However, the length of these cycles isn’t static—they can vary across time, and, with these fluctuations, your monthly fertility levels. These variations are responsible for the slim possibility of conceiving while you’re on your period.

How Can You Get Pregnant on Your Period?

In the US alone, some 14 to 25% of women experience irregularities in their menstrual cycle. These rhythms can make ovulation and menses hard to track, which means the likelihood of conceiving during periods may be elevated.

Two tricks of ovulatory timing are responsible for the possibility of conceiving while menstruating.

#1 The Long Menstrual Phase

Let’s take an example. Say you have a long menstrual period that follows this pattern:

• Menstruation – Days 1 to 8
• Follicular – Days 1 to 11
• Ovulation – Day 12
• Luteal – Days 13 to 26

If you have a long menstrual cycle (8 days) with a regular follicular phase, and have sex on the last day of your period, there is a small chance that a sperm cell could survive until ovulation—resulting in pregnancy.

#2 The Shortened Follicular Phase

Sometimes, a woman’s follicular phase (the time from menstruation to ovulation) can be shorter. This also gives sperm cells a greater chance of surviving between having sex during your period and the day you ovulate.

Why would a follicular phase shorten? A few reasons:

• Age – As you age, your body’s egg supply and quality begin to dwindle, a process that eventually triggers menopause. Transitioning into menopause can affect ovulation times, leading to irregular or shortened cycles. So while your overall fertility will decrease, the likelihood of conceiving during your period may actually increase.
• Weight – Being under or overweight can also cause ovulation to occur too early. One study of over 60,000 menstrual cycles found women with a BMI exceeding 35 had a higher incidence of cycle irregularity. On the other hand, those who are underweight may also experience hormonal imbalances which can lead to cycle irregularity.
• Lifestyle – Lifestyle factors like diet, substance overuse, and stress can all induce early or irregular ovulation. Research suggests that acute as well as chronic stress may trigger early ovulation.
• Irregularity – Even if your period arrives every 28th day on the dot, that doesn’t mean ovulation keeps such a strict schedule. One study found that only 30% of women had a high-fertility ovulation window on their expected days across their cycle. Sometimes, your ovulatory cycle is just unpredictable!

How likely are you to get pregnant during your period?

In short: if the final days of your period overlap with the 5 days before ovulation, then it’s important to use birth control if you don’t want to get pregnant. Sure, those period days aren’t the peak of your fertility, but they can still carry a near 10% chance of conceiving in some individuals.

Can you get pregnant on your period with birth control?

The short answer? Yes—but it’s also highly unlikely.

Some hormonal birth control methods offer more than 99% effective protection against unwanted pregnancies. That said, there is no such thing (yet) as a birth control method that’s 100% fool-proof. Largely due to human error, you can get pregnant while using birth control, whether you’re menstruating or not.

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