March Fertility

Why Spring Is Nature's Peak Conception Season

I've noticed something curious over the years: every March, like clockwork, my body feels different. My libido kicks into a higher gear. My skin looks clearer. I have more energy, even before I've had my morning coffee. And my cervical mucus—yeah, we're going there—becomes noticeably more abundant and fertile-quality, even when I'm not anywhere near ovulation according to my tracking app.

For a while, I thought I was imagining it. Maybe it was just the relief of winter ending, or the psychological boost of longer days. But then I started paying attention to the pattern, year after year. March through May, my body acts like it's ready to make a baby, whether I am or not. My cycles become more regular. Ovulation is stronger, more obvious. Even my moods stabilize in a way they don't during other seasons.

Turns out, I'm not imagining anything. Human fertility has a seasonal pattern, and spring—particularly late winter through early summer—is when our bodies are biologically primed for conception. This isn't woo-woo or astrology. It's evolutionary biology, backed by decades of demographic data and endocrinological research. Our modern lives might run on artificial light and climate control, but our bodies are still following ancient seasonal rhythms that we've mostly forgotten about.

Understanding this matters whether you're trying to conceive, actively preventing pregnancy, or just trying to make sense of why your body feels the way it does at different times of year. So let's talk about what's actually happening inside your body when spring arrives, and how you can work with—or against—these fertility signals depending on your goals.

The Data: Spring Babies Are Real

If you look at birth rate data across populations in the Northern Hemisphere, there's a consistent pattern: more babies are born in late summer and early fall (August through October) than any other time of year[1]. Work backward nine months, and you land squarely in late fall through early spring—with March showing up repeatedly as a peak conception month.

This pattern holds across different cultures, geographic locations, and time periods, which suggests it's not just about holiday romance or cultural factors. It's biological. Even in modern societies with artificial lighting, temperature control, and year-round food availability, we still see these seasonal fertility fluctuations. They're muted compared to what they would have been for our ancestors, but they're there.

In equatorial regions where seasonal variation is minimal, birth rates are more evenly distributed throughout the year. But the further you get from the equator—where seasonal changes in daylight, temperature, and food availability are more pronounced—the stronger the spring fertility surge becomes. This is a clue that we're dealing with evolutionary adaptations tied to environmental cues.

Evolutionary Logic: Why Spring Makes Sense

From a survival perspective, spring conception makes perfect sense. If you conceive in March or April, you're giving birth in December or January—right after the harvest, when food stores would traditionally be at their peak. The mother would have had access to abundant nutrition during the critical third trimester (summer and fall), when fetal growth is most rapid. The baby would be born during a time when the family unit would be more stationary (winter), allowing for bonding and nursing without the demands of planting or harvesting.

By the time spring returned and resources became scarce again (the "hungry gap" between winter stores running out and new crops coming in), the baby would be several months old, more robust, and the mother would be past the most physically demanding postpartum period. It's elegant timing.

Our bodies evolved to optimize for this pattern through hormonal responses to environmental cues—primarily light exposure and temperature changes. Even though most of us aren't living agrarian lifestyles anymore, our endocrine systems are still responding to these signals.

Photoperiod and Melatonin: The Light Connection

The primary environmental cue driving seasonal fertility changes is photoperiod—the length of daylight. As days get longer from winter solstice through summer solstice, your pineal gland registers the increased light exposure and adjusts melatonin production accordingly.

Melatonin isn't just a sleep hormone. It's also a reproductive regulator. In darkness (winter), melatonin levels are higher and sustained for longer periods, which has a suppressive effect on gonadotropin-releasing hormone (GnRH) from the hypothalamus. GnRH is the master hormone that triggers the entire reproductive cascade—it tells your pituitary to release LH and FSH, which then tell your ovaries to mature follicles and ovulate.

When daylight increases in spring, melatonin production decreases. This disinhibits GnRH, allowing for stronger, more consistent ovulatory signals. The result: more robust follicular development, higher quality eggs, better progesterone production after ovulation, and overall improved fertility[2].

This is why light exposure matters so much for hormonal health. Women who work night shifts or who have minimal daytime light exposure (think: living in northern latitudes during winter, or spending all day indoors) often experience menstrual irregularities, anovulatory cycles, and fertility challenges. Their bodies aren't getting the light cues needed to maintain optimal reproductive function.

Interestingly, the specific wavelength matters. Blue light exposure in the morning (which is abundant in natural sunlight) has the strongest effect on suppressing melatonin and signaling "daytime" to your circadian system. This is why getting outside in the morning, especially in spring when days are rapidly lengthening, can have such a profound effect on your cycle regularity and fertility.

Temperature, Thyroid, and Metabolic Shifts

Seasonal temperature changes also play a role in fertility fluctuations. As ambient temperature increases in spring, your body's metabolic demands shift. Thyroid activity—which governs metabolic rate—tends to be higher in winter to maintain body temperature, then decreases somewhat as external temperatures rise.

This might seem counterintuitive, but slightly lower thyroid activity in spring and summer actually correlates with improved fertility in some studies. The theory is that during winter, metabolic resources are diverted toward thermogenesis (heat production) at the expense of reproductive function. When that metabolic burden lifts in spring, those resources can be redirected toward ovarian function and reproduction[3].

This doesn't mean hypothyroidism is good for fertility—it's not. But there's a nuanced seasonal variation in optimal thyroid function, and your body naturally adjusts across the year. Women with thyroid disorders often notice their symptoms and medication needs fluctuate seasonally, which reflects these underlying metabolic shifts.

Temperature also affects sperm production in men. Sperm quality tends to be highest in late winter and early spring, when testicular temperature regulation is easier. Combined with increased female fertility in spring, you have both partners operating at peak reproductive capacity simultaneously—another evolutionary advantage.

Vitamin D: The Sunshine Fertility Hormone

Vitamin D levels are another major piece of the spring fertility puzzle. After months of winter with limited sun exposure, most people's vitamin D stores are at their lowest by late winter. As sun exposure increases in spring, vitamin D levels rise—and this has direct effects on reproductive health.

Vitamin D receptors are found in the ovaries, uterus, and placenta. Adequate vitamin D is necessary for:

• Healthy follicular development

• Proper estrogen and progesterone synthesis

• Endometrial receptivity (the uterine lining's ability to accept an embryo)

• Immune regulation at the maternal-fetal interface

• Calcium metabolism, which affects egg quality

Women with vitamin D deficiency have higher rates of anovulation, irregular cycles, and fertility challenges[4]. The spring increase in vitamin D production as sun exposure increases likely contributes to the observed fertility surge.

This is one reason why intentionally getting morning sun exposure in spring—face, arms, legs, without sunscreen for 15-30 minutes—can be a game-changer for hormonal health. You're not just boosting vitamin D; you're also getting the circadian light exposure that regulates melatonin and supports healthy hormone rhythms.

Cervical Mucus and Seasonal Patterns

If you've ever tracked your cervical mucus for fertility awareness or natural family planning, you might have noticed that the quality and quantity varies not just throughout your cycle, but also across seasons. This isn't in your head.

Estrogen stimulates cervical mucus production, and the spring surge in reproductive hormones means you may produce more abundant, higher-quality fertile mucus even outside your immediate fertile window. This mucus—clear, stretchy, and slippery like raw egg white—is designed to help sperm survive and travel through the reproductive tract.

From a natural family planning perspective, this means spring can be trickier to navigate if you're trying to avoid pregnancy. You might see more days of fertile-quality mucus, making it harder to identify your true peak fertility. On the flip side, if you're trying to conceive, this abundance of fertile mucus is working in your favor, creating a more sperm-friendly environment over a longer window.

Some women also notice their libido tracks with these seasonal mucus changes. Estrogen doesn't just affect cervical mucus—it also influences sex drive, energy levels, and mood. The spring estrogen surge can mean feeling more interested in sex, more energetic, more socially engaged. Your body is literally trying to get you to reproduce when conditions are optimal.

Working With Spring Fertility: For Those Trying to Conceive

If you're actively trying to get pregnant, spring is your ally. Here's how to maximize your natural fertility advantage:

Prioritize morning light exposure: Get outside within an hour of waking, ideally for 20-30 minutes. No sunglasses. This optimizes your circadian rhythm and supports healthy hormone production.

Optimize vitamin D: Get your levels tested. Aim for 40-60 ng/mL (not the bare minimum "sufficient" level of 30 ng/mL). Combine sun exposure with supplementation if needed, especially if you're coming out of winter with depleted stores.

Support your luteal phase: The spring fertility surge is great for ovulation, but you still need adequate progesterone after ovulation to support implantation. Make sure you're eating enough—undereating and high stress both suppress progesterone. Consider seed cycling or supplemental progesterone if you have a history of short luteal phases or low progesterone.

Track consistently: Use a combination of basal body temperature, cervical mucus, and optional LH testing to identify your fertile window. Spring cycles often show stronger ovulatory signals (higher temperature shift, more obvious mucus changes), making tracking easier.

Don't overthink timing: The fertility window is the 5-6 days leading up to and including ovulation day. Aim for every other day during this window. More frequent isn't necessarily better—sperm quality can decrease with very frequent ejaculation.

Reduce endocrine disruptors: Everything we talked about in the Easter lily post applies here. Spring cleaning products, synthetic fragrances, pesticides on conventional produce—minimize these exposures to reduce hormonal interference.

Working Against Spring Fertility: For Those Preventing Pregnancy

If you're using natural family planning methods or fertility awareness-based methods (FAM) to prevent pregnancy, spring requires extra vigilance.

Expect more fertile days: Your fertile window might be longer than usual. Don't assume your pattern from winter months will hold. Track carefully every cycle.

Confirm ovulation: Use temperature tracking to confirm that ovulation actually occurred, rather than just assuming it did based on calendar dates or apps. Spring cycles might ovulate earlier or later than your typical pattern.

Barrier methods during uncertainty: If you're seeing fertile-quality mucus but aren't sure if you're actually approaching ovulation, use barriers or abstain. Better safe than surprised.

Consider your libido: If your sex drive increases in spring (which is common), factor this into your method choice. Abstinence-based approaches might feel harder to stick to when your body is pushing you in the opposite direction.

Backup method: If pregnancy would be a serious problem, consider layering FAM with another method (condoms, diaphragm, etc.) during your fertile window for extra security.

Don't trust apps blindly: Fertility apps that predict ovulation based on averages are notoriously unreliable, especially during seasonal transitions. Use them as a guide, not gospel.

Honoring Your Body's Seasonal Signals

There's something powerful about recognizing that your body is still attuned to ancient rhythms, even in our modern, climate-controlled, artificially-lit world. You might not be planning your conception around harvest cycles, but your endocrine system is still responding to the lengthening days and warming temperatures the same way it has for thousands of generations.

This awareness can shift how you relate to your body. Instead of fighting against fluctuations in energy, libido, or mood across the year, you can recognize them as normal, adaptive responses to environmental changes. Instead of wondering why you feel more vital and alive in spring, you can understand that your body is literally designed to feel that way.

For some women, this knowledge is empowering. It validates experiences they've been having but couldn't quite name. For others, it's practical information that helps them make better decisions about fertility management, whether they're trying to conceive or prevent pregnancy.

And for those of us just trying to understand our own bodies better, it's a reminder that we're not separate from nature. We're part of it. Our hormones, our cycles, our fertility—they're all connected to the earth's rhythms in ways that are easy to forget when you live in a city and work under fluorescent lights.

What If Your Cycles Don't Follow This Pattern?

Not everyone experiences a spring fertility surge, and that's worth acknowledging. If you have PCOS, hypothalamic amenorrhea, thyroid disorders, or other hormonal conditions, your cycles might not respond to seasonal cues the way they theoretically should. You might be anovulatory year-round, or your ovulation might be triggered more by stress levels, body weight changes, or other factors than by photoperiod.

This doesn't mean something is wrong with you. It just means your system is dealing with other inputs that override the seasonal signals. Understanding the baseline pattern can still be useful, though, because it gives you a framework for what optimal might look like and what environmental factors you can manipulate to support better hormonal function.

For example, if you know that increasing light exposure in spring should theoretically support better ovulation, you can experiment with prioritizing morning light even if you're not ovulating regularly. You might find that combining light exposure with other interventions—adequate food intake, stress management, targeted supplementation—helps restore more regular cycles.

So here's what I'm doing differently this spring: I'm paying attention. I'm noticing when my body shifts into that higher-fertility gear, and instead of being confused or frustrated by it, I'm recognizing it for what it is—an evolutionary response to environmental cues that have meant "time to reproduce" for millennia.

I'm getting outside more, first thing in the morning, because I know the light exposure supports my hormonal health in ways I can feel. I'm tracking my cycles with curiosity rather than anxiety, watching how the spring months change my patterns. And I'm adjusting my natural family planning approach accordingly, because I know my fertile window is likely longer and more robust right now than it will be in November.

Most of all, I'm appreciating that my body knows what it's doing, even when my conscious mind doesn't have the full picture. The spring fertility surge isn't random. It's intelligent. It's adaptive. And whether I'm working with it or against it, understanding it gives me agency over my own reproductive health.

That feels like power worth having.

References

[1] Rojansky, N., Brzezinski, A., & Schenker, J. G. (1992). Seasonality in human reproduction: an update. Human Reproduction, 7(6), 735-745.

[2] Wehr, T. A. (2001). Photoperiodism in humans and other primates: evidence and implications. Journal of Biological Rhythms, 16(4), 348-364.

[3] Danilenko, K. V., Sergeeva, O. Y., & Verevkin, E. G. (2011). Menstrual cycles are influenced by sunshine. Gynecological Endocrinology, 27(9), 711-716.

[4] Lerchbaum, E., & Obermayer-Pietsch, B. (2012). Vitamin D and fertility: a systematic review. European Journal of Endocrinology, 166(5), 765-778.