LH surge: What is it, and why is it important if you’re TTC?

You might have already heard about the roles that estrogen and progesterone play in your menstrual cycle. When we talk about all things periods and reproductive health, they’re arguably the two that get mentioned most. But they’re not alone. Two other hormones play a really crucial role in keeping your cycle moving. They’re called follicle-stimulating hormone and luteinizing hormone (LH), and without them, you wouldn’t ovulate.

So, let’s break down the role of LH, from when it might rise in your cycle, how you can spot this using ovulation kits, and why it’s important to understand LH if you’re trying to conceive (TTC). 

Key takeaways about the LH surge before ovulation

• Luteinizing hormone is a hormone produced in the brain’s pituitary gland and triggers really important processes during your cycle, such as ovulation and supporting your body’s changes during early pregnancy. 
• It surges around 28 to 36 hours before ovulation, triggering one of your ovaries to release a mature egg. 
• You can track these changes using ovulation predictor kits (OPKs). OPKs work by detecting the rise in your LH levels around 24 hours before ovulation. 
• You can also predict when you might ovulate and when your fertile window is using an app like Flo. You can input the results of your ovulation tests as well as any changes to your discharge and basal body temperature. 

What is an LH surge? 

Like estrogen and progesterone, LH is another important hormone that surges during your cycle to trigger processes like ovulation. 

To better understand what an LH surge is, it can help to break down your cycle into two phases. 

The first part of your cycle is known as the follicular phase. It begins on the first day of a new period and runs until the day before one of your ovaries releases an egg. During this time, your body releases follicle-stimulating hormone. This hormone prompts your ovaries to start to develop and grow follicles (small fluid-filled sacs where an egg can develop and mature). 

Once the egg is ready and mature, your body releases a surge of LH. This triggers the egg to be released from the follicle, out of your ovary. This is known as ovulation, and while it isn’t an official “phase” in your cycle, you can think of it as an important stand-alone event. 

Once you’ve ovulated, you enter the second phase of your cycle, the luteal phase. LH continues to be released at a lower level alongside progesterone, which thickens the lining in your uterus, preparing it for a potential pregnancy. 

How long after an LH surge do you ovulate? 

Typically, it takes between 28 and 36 hours after the beginning of the LH surge for an egg to break free from its follicle and enter the fallopian tube (ovulation). If you have unprotected sex during this time, this is when the egg might be fertilized by a sperm.

If you’re tracking when you might ovulate, then it’s important to remember that while your LH surge is detectable in the bloodstream “immediately,” says Dr. Sara Twogood, obstetrician and gynecologist, US, “it takes about 12 hours” for the spike in LH levels to be detectable in the urine. This means a home ovulation predictor kit will detect the LH surge “about 24 hours before ovulation.” 

How to test for an LH surge 

Ovulation isn’t always easy to predict. The day you ovulate can change from cycle to cycle. However, it isn’t impossible, and there are some tried and tested methods: 

Ovulation predictor kits

OPKs work like pregnancy tests, testing for hormones in your urine. Instead of looking for pregnancy hormones, though, they are looking for an increase in LH levels. 

If you know your usual cycle length, start testing for ovulation around the eighth day. Test every day around the same time. First thing in the morning is a good time because ideally, you won’t have urinated for four hours before you test. Depending on which brand you use, a positive LH test might have two lines (like a pregnancy test) or a smiley face. 

It’s fiddly, but worth doing. A study published in 2023 found that timing sex using ovulation tests increased the chances of pregnancy by between 20% and 28% per year, compared with 18% without using the tests.

And if you’re unsure of your cycle length, you can use an app like Flo to help you understand when the best time for you to start testing for ovulation is. 

Other methods to track ovulation

Although OPKs are an easy way to track ovulation, Dr. Twogood says, “I usually tell my patients to do two methods.” Monitoring your body for signs that ovulation is coming — known as fertility awareness — can help you to predict your fertile days. 

Tracking changes in your basal body temperature

Basal body temperature (BBT) is your temperature when your body is completely at rest. When you ovulate, your BBT increases by 0.4ºF to 1°F (0.22ºC to 0.56ºC). So, measuring your BBT every day using a special thermometer with two decimal places can help you learn when you tend to ovulate. You can input your BBT data into the Flo app to get even more accurate ovulation predictions. 

Tracking changes in your cervical mucus 

Just before you ovulate, your discharge (also known as cervical mucus) goes through some changes. Throughout the first half of a cycle, cervical mucus is typically thick, white, and dry. Then, just before and during the day of ovulation, it turns clear and slippery, a bit like raw egg whites. This is to help make it easier for sperm to swim through the reproductive tract and meet an egg. Clever, right? 

This egg white discharge usually lasts for about four days, signaling that it’s time to have unprotected sex if you’re trying. After ovulation, discharge typically becomes dry until you get your period a couple of weeks later. 

How to use ovulation timing after the LH surge when TTC

If you’re trying to get pregnant, knowing when your LH levels are surging can provide you with a guide on when the best time might be to have conception sex. Dr. Twogood says if you have regular cycles, you may be able to identify the six or seven days during which you are at peak fertility simply by consulting a calendar. “The optimal timing of sex is every day or every other day in this window,” she says. 

She adds that an LH urine test will “help predict the day before ovulation and the day of ovulation. These are the peak fertility days, and it is optimal to have sex both of these days.”

This can be really handy if you’re trying for a baby, but all this planning can seem to zap the fun out of conception sex. Try not to let all the planning take the fun out of it. Having specific days in mind may amp up the pressure or take some of the spontaneity out of the situation. If you want to try to conceive this cycle, try setting up date nights. Build intimacy around these times with your partner and, remember, if you aren’t in the mood or need to take a step back, that’s absolutely OK. 

Tips to maximize fertility 

When it comes to fertility, there are a lot of myths, but most of them don’t stand up to scrutiny. You don’t need, for example, to bother putting your legs up against the wall after sex or eating oysters. Neither has been proven to help fertility. 

Instead, follow the usual healthy diet and exercise advice. Focus on drinking plenty of water every day, which can improve the quality of fertile cervical mucus. Give up smoking and cut down on alcohol and caffeine. It can also help to eat leafy greens, citrus fruits, and pulses, all of which contain folic acid, and are part of a Mediterranean-style diet. Finally, take a supplement that contains at least 400 mcg of folic acid. 

Dr. Twogood adds, “Nutrition and diet is a longer-term strategy to optimize overall health that can help optimize fertility, not just at the time of ovulation.”

If you’d like more hints and tips on how to track ovulation, you can use Flo’s ovulation-tracking app. It may help you to understand your fertile window better and offer you personalized cycle predictions so you feel more prepared.