Getting pregnant - What has to work for it to happen?

Fertility is a nuanced and complex process. Understanding more about what goes on in our bodies and how pregnancy actually happens will help you gain more perspective on what you can do to optimize your chances.

The short story is: you need one healthy egg and one healthy sperm. They need to be able to meet in your fallopian tube so that fertilization can happen. When fertilized, the egg becomes an embryo which is carried through the tube down to an endometrial lining in the uterus where it can implant. If it does so successfully, that’s when you’re considered pregnant.

To summarize, several events and body parts need to work as they should. And to make it more complicated - each event must be supported by the right hormone, in the right amount, at the right time. If any element is unhealthy or missing, a healthy pregnancy cannot take place. 

When should I have sex to become pregnant?

When you’re trying to become pregnant, you may often wonder when you should have sex to maximize your chances. That’s the right question to ask because timing is everything. You have to have sex within the woman’s so-called “fertile window” for pregnancy to be possible. 

The fertile window is a six-day window per cycle - 5 days before ovulation and 1 day after. Women often think that day of ovulation is the start of the fertile window, but as you read more about below, the most fertile days are in fact found a few days before ovulation. 

Identifying these days may sound simple enough, but it can require some patience and a bit of calculation. There are several ways to do it and understanding your cycle is fundamental. Let’s start by going through the phases of the menstrual cycle, and how they relate to pregnancy planning.

The menstrual cycle: Identifying your fertile window

The menstrual cycle exists for the sole reason of helping you get pregnant. Whether you want to or not, the body prepares itself every month like clockwork if it works as it should.

The exact length of the menstrual cycle varies from person to person, but the cycle always consists of two different phases; the follicular phase and the luteal phase. The follicular phase begins with your period and ends with ovulation, and the luteal phase begins with ovulation and ends with your next period.

Follicular phase 

When a new cycle starts a number of follicles containing egg cells start to mature. The maturing follicles release estrogen, which helps build up the uterine lining and makes it ready to receive an embryo if an egg is fertilized.


As the estrogen levels surge, it stimulates the production of the gonadotropin-releasing hormone (GnRH), which then stimulates the pituitary gland to secrete luteinizing hormone (LH). 

The increase in LH leads to one of the follicles bursting and releasing a mature egg cell. Ovulation tests usually look for this LH peak to identify that ovulation could occur within 24-48 hours.

A normal ovulation cycle lasts for about 24 hours. Once a mature egg has been released, it has only 12-24 hours before it dies or dissolves if not fertilized. 

But here’s the thing - sperm, on the other hand, can survive up to five days in the fallopian tubes. They can stay in the uterus, and make their way up to the fallopian tube before an egg is released. This is why the fertile window begins before ovulation happens and not after.

In short: having sex before ovulation will help you because it means that there will be sperm already lying in wait when an egg is released, which increases your chances of success.

Luteal phase

After ovulation, fertilization can happen if the egg meets a sperm to form an embryo, and the embryo then travels down towards the uterus. When an egg is released, the ruptured follicle produces a progesterone hormone. This hormone helps the endometrial lining gain the right structure to provide a good environment for the embryo.

Then either one of two things happen. If an embryo implants in the endometrial lining, a pregnancy will start to develop and hormone levels such as estrogen and progesterone will remain high. 

However, if fertilization or implantation has failed, then hormone levels will drop and the lining that has been built up during the cycle collapses. The lining is bled out, you get your period and the whole process starts all over again.

Are the chances equally high throughout the entire fertile window?

A study from the New England Journal of Medicine shows that if sex happened five days before ovulation, the probability of a successful pregnancy is 10%. If sex happens on the day of ovulation, that probability goes up to 36%. 

Most fertile days are found just a few days before ovulation so there’s no reason to have sex only on the one ovulation day when you’re trying to conceive. 

Tracking ovulation can be emotionally exhausting, especially after many cycles and even more so if ovulation is irregular. If it feels overwhelming, having sex every other day is a common recommendation.

How often should I have sex?

You may have come across advice that having sex too often could lead to a low sperm count. However, trying to “save up” and only having sex once during a perfectly timed moment could mean that you risk missing the fertility window altogether.

New studies also show that having sex often actually improves sperm quality. IVF clinics used to recommend a few days of abstinence before leaving a sperm sample. Today, that’s changed and no more than two days of abstinence is usually recommended to maximize sperm quality.

How do I track ovulation?

Given that timing is everything when you are trying to conceive, it makes sense to track your menstrual cycle and ovulation to time sex correctly. 

There are several ways to do it, but what all the methods have in common is that they entail some form of measuring or tracking either hormone levels or things happening in your body.

On top of the methods below, you can also choose to visit your doctor and measure hormone levels via blood tests, or ultrasounds to look at your developing follicles. However, visits to the doctor can be tedious and expensive and best done only if you are going through a form of fertility treatment.

At-home Ovulation Predictor Kits (OPKs)

  • What is it?

An at-home OPK normally measures LH levels found in urine as a spike in LH indicates that ovulation will happen with 24-48 hours. Sometimes, they also measure estrogen as rising estrogen levels precede a rise in LH. If the estrogen level detected by the test is high enough, it indicates that the fertile window has started.

  • How do you use it?

OPKs are similar to at-home pregnancy tests — you simply urinate on the test stick. They are also widely available, you can get them over the counter at drugstores, pharmacies, and even major supermarkets without a prescription.

  • Important to note:

Identifying a peak in LH indicates that ovulation may happen, but it is not irrefutable proof that ovulation will definitely occur.

OPKs also might not be for everyone. For example, women with polycystic ovarian syndrome (PCOS) tend to have constantly heightened levels of LH and get false positives. Women taking hormones to stimulate follicle growth and ovulation may also have higher LH levels.

The tests have high reliability if you don’t belong to one of these groups, but the prices can add up if you have to keep using them for many cycles. 

Tracking temperature

  • Why?

When progesterone is released by the ruptured follicle after ovulation, it causes your body temperature to rise. This is why tracking your basal body temperature (BBT) at the exact same time every morning can help you identify that you have already ovulated. By charting the rise and fall of your BBT during a menstrual cycle, you can predict your fertile window based on your previous cycles.

  • How?

It’s preferable to use a basal thermometer showing you the temperature in tenths of a degree as this allows you to note small changes in body heat. There are also wearables that track your BBT which can be practical if you have a hard time adjusting your daily routines to measuring your temperature at the same time every day.

  • Important to note:

As mentioned before, you are the most fertile before ovulation and your BBT won’t rise until after it has already happened. So if you use the method to become pregnant it requires a regular cycle to be useful. 

Tracking cervical mucus

  • Why?

When estrogen levels peak before ovulation, it affects the appearance and sensation of cervical mucus. Knowing what type of cervical mucus to look out for can help you identify your fertile window.

  • How?

Checking cervical mucus might not be comfortable for some women, so it might take some time to get to know your body. Some women observe cervical mucus on toilet paper, but it is also possible to observe the discharge on underwear or to put clean fingers into the vagina and then check the color and texture there.

Thick, creamy, and yellowish discharge that is not stretchy is transitional mucus that indicates intermediate fertility. What you’re looking for is transparent, watery, and stretchy mucus which indicates high fertility.

  • Important to note:

Similar to the limitations of tracking your BBT, it can be less reliable to track your cervical mucus to predict ovulation based on your previous cycles because not everyone has a regular cycle.


Apps are a way for you to keep track of your hormone levels, temperature, mucus, and so on. It also helps you visualize patterns. You can use Tilly to do this, for example. 

If an app offers predictions, remember that is only a guess based on your previous cycles. The prediction can very helpful but there's no guarantee that one cycle looks like the one before. Understanding the changes in your body and measuring hormone levels is always more secure than relying on the recommendation on an app. 


Evangelia Elenis, MD, PhD.

This text is fact checked by Evangelia Elenis, MD, PhD. Dr. Elenis is a chief physician in Obstetrics and Gynecology, and a subspecialist in Reproductive Medicine. She is a PhD and affiliated researcher at Uppsala University with postdoctoral studies at Harvard Medical School.

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