What is progesterone’s role in reproduction?

For women, progesterone plays a multi-faceted role in your overall health. However, it has one responsibility that’s decisive in determining your fertility: thickening the uterine lining.

Whether you’re actively trying to conceive or are already pregnant, your uterine lining (otherwise known as the endometrium) must be dense to support a baby’s growth. It’s this tissue that creates a “landing pad” for a fertilized egg to implant in and become an embryo. Without sufficient progesterone, the uterine lining will not be thick enough to conceive or maintain a pregnancy.

Aside from building up the uterine lining, progesterone has a few other crucial roles in the pregnancy journey:

• Keeping other reproductive hormones (e.g. estrogen) in balance
• Developing your mammary glands for breastfeeding
• Promoting good mood, sound sleep, and healthy blood pressure

Maintaining a healthy and normal progesterone level isn’t just a matter of volume—impeccable timing is equally important.

Progesterone and Menstruation

Fertility is driven by the menstrual cycle, which relies heavily on progesterone—in fact, that extra tissue in the uterine lining is what produces your period each month.

Every month, vaginal progesterone rises, causing the uterine lining to thicken in preparation for a healthy, fertilized egg to implant. Over the four menstrual stages, healthy female progesterone levels usually follow the following pattern:

• The follicular phase – Your follicular phase starts right after your period ends. This phase lasts until ovulation, when a mature egg is dispatched by an ovary. During this time, progesterone levels are relatively low—approximately 0.89 ng/ml.
• Ovulation – Some 5 to 12 days after your period ends, the ovaries release a mature egg into the Fallopian tubes. Ovulation triggers the corpus luteum (which once contained the mature egg) to boost progesterone levels, which escalate to between 1.2 ng/ml and 12 ng/ml.
• The luteal phase – This is when progesterone kicks into high gear. Levels skyrocket to around 24 ng/ml, which stimulates the endometrium to thicken in preparation for a fertilized egg. Elevated progesterone also encourages estrogen production during this phase (thereby creating those lovely PMS symptoms).
• Menstruation – If an egg doesn’t implant, the corpus luteum disintegrates, halting progesterone production. As a result, progesterone levels suddenly drop to 0.89 ng/ml, which causes the uterine lining to shed—aka, your period.

Progesterone and Pregnancy

If an egg fails to implant, it triggers a low progesterone level and a monthly visit from Auntie Flo. However, if an egg does implant, your body begins a new journey: pregnancy.

After implantation, rising hCG (human chorionic gonadotropin) levels tell your corpus luteum to keep producing progesterone. The placenta (the organ connecting your baby to your uterus) will take over progesterone production once it’s fully formed during the luteal-placenta shift, which is usually around the mid-first trimester.

Over that time, progesterone volume continues to elevate and stay high, hitting the following levels at each trimester on average:

• First trimester – 11 to 44 ng/ml
• Second trimester – 25 to 83 ng/ml
• Third trimester – 58 to 214 ng/ml

Far and away the most important role progesterone plays during pregnancy is maintaining a thick uterine lining. A thick endometrial tissue supplies your baby with the nutrients and physical cushion it needs to grow.

High progesterone levels also play a few additional roles during pregnancy:

• Expanding your uterus to reduce contractions and premature birth
• Boosting blood flow to the pelvis to supply fetal nutrients
• Strengthening pelvic floor muscles for easier labor and delivery
• Firing up mammary glands to prepare for breastfeeding

How does low progesterone affect fertility?

When progesterone falls below healthy levels, the body isn’t able to create a hospitable environment for implantation. If a fertilized egg can’t implant in the uterus, the body won’t support a pregnancy.

Unfortunately, existing research on low progesterone is scarce. Some studies show that fertility treatment options like progesterone therapy can improve conception outcomes in clinical pregnancies and frozen embryo transfer.

Low Progesterone Symptoms

If you’ve been struggling to get pregnant, your healthcare provider can help investigate if low progesterone is to blame. In the meantime, here are some associated symptoms to look out for:

• Exaggerated PMS symptoms
• Headaches
• Anxiety and depression
• Insomnia or sleeping issues
• Irregular menstrual cycles
• Bloating
• Low sex drive
• Weight gain
• Breast tenderness

Keep in mind, some of these symptoms are indirectly related to low progesterone. When progesterone levels are depressed, it impairs the system of checks and balances that keeps estrogen levels in check. This condition is known as estrogen dominance, and it can result in weight gain, breast tenderness, and a low sex drive.

How does low progesterone affect pregnancy?

Low progesterone levels can be frustrating when you’re trying to get pregnant, and they can be downright dangerous during pregnancy.

Remember, as the hormone that keeps your little one safe, swaddled, and healthy in your uterus, progesterone is indispensable for delivering adequate nutrients to your baby. Research shows that low progesterone correlates with the following during pregnancy:

• Spotting – By about the 8th week of pregnancy, the lutea-placental shift occurs between your corpus luteum and placenta. This can temporarily slow progesterone levels, triggering vaginal bleeding (similar to a period). If you do experience bleeding during pregnancy, contact a medical provider to double-check that your baby is safe.
• Miscarriage – Research on progesterone and risk of pregnancy loss is complex. Over two dozen studies show that low progesterone levels correlate with non-viable pregnancies, particularly when serum levels dive below 35 nmol/L. Fortunately, research also shows that progesterone therapy can reduce miscarriage rates in pregnant women with early bleeding.
• Ectopic pregnancy – Research shows that low progesterone could be an effective serum marker for ectopic pregnancy risk. Ectopic pregnancies occur when the fetus develops outside of the uterus, which can occur in 1 out of 50 pregnancies. This condition is life-threatening to both the mother and the baby, requiring a medically terminated pregnancy, so contact your medical provider immediately if you experience persistent pelvic pain while pregnant.

What causes low progesterone?

Hormones, especially sex hormones, are a complicated and delicate system, and it can be difficult to pinpoint the precise factors that can throw them off kilter. At times, even small lifestyle changes like introducing exercise or improving sleep can have a major biological butterfly effect.

That said, there are some causes commonly thought to be behind low progesterone:

• Polycystic Ovarian Syndrome (PCOS) – PCOS is estimated to affect up to 12% of women. This condition causes excess androgens (male sex hormones) in female bodies. It can lead to anovulation (lack of ovulation), which depresses both progesterone and overall fertility.
• Stress – Both mental stress and physical stress can boost the brain’s production of cortisol (a stress hormone), which disrupts reproductive hormones and lowers the likelihood of conception. Progesterone is considered the hormonal precursor to cortisol—which means that the more stress experienced, the more progesterone the body expends to create cortisol. This decreases the amount of progesterone available to support a healthy conception and pregnancy.
• Body weight – Being overweight or underweight can lead to low progesterone. In obese women, excess fat cells create more estrogen, which can disrupt the estrogen-progesterone balance. Meanwhile, underweight women are much more likely to experience anovulation and low fertility than women at a healthy weight.
• Thyroid conditions – Low thyroid function (or hypothyroidism) directly lowers progesterone production. Studies show that low levels of thyroid hormones, like T3, T4, and TSH, can disrupt the development of reproductive tissues in women.
• Menopause – Menopause, which typically sets in your 40s, signals the end of your body’s ovulation and menstruation cycles. At this stage, progesterone dips, which can come with symptoms like hot flashes, vaginal dryness, and night sweats.

Whether you’re trying to get pregnant or just feel like something’s “off,” it can be unnerving (and frustrating) to pinpoint which causes are behind these setbacks. But with determination, patience, and plenty of self-compassion, you’re taking the necessary steps to care for yourself—which is the first step in caring for a future little one.

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