How the Menstrual Cycle Impacts Fertility: Understanding the Phases
By Dr Greg Jenkins — Sydney-based specialist in Fertility, Obstetrics and Gynaecology
The menstrual cycle is not only a monthly routine for many women but also a key player in fertility. Each phase of the cycle represents a carefully coordinated hormonal process that prepares the body for potential pregnancy. Understanding how the cycle works and how it influences fertility is crucial for those trying to conceive. In this article, we’ll break down the four phases of the menstrual cycle, how they impact your fertility, and what you can do to maximise your chances of conception.
Key Takeaways:
The menstrual cycle typically ranges from 21 to 35 days, not just the commonly cited 28 days.
Four key hormones regulate the cycle:
FSH stimulates follicle growth.
LH triggers ovulation.
Oestrogen thickens the uterine lining.
Progesterone prepares the uterus for pregnancy.
The cycle has four phases:
Menstrual Phase (Days 1-5): Shedding of the uterine lining.
Follicular Phase (Days 6-14): Follicle growth and rising fertility.
Ovulation Phase (Around Day 14): Egg release and peak fertility.
Luteal Phase (Days 15-28): Uterus prepares for potential pregnancy.
The fertile window is the 5 days before and including ovulation.
Factors like stress, diet, and age can affect cycle regularity and fertility.
The Basics of the Menstrual Cycle
The menstrual cycle is a series of hormonal changes that prepares a woman's body for potential pregnancy each month. While we often hear that a cycle is 28 days long, the reality is that anywhere from 21 to 35 days is considered normal. In fact, according to a comprehensive study published in npj Digital Medicine in 2019, which analysed over 600,000 menstrual cycles, only 13% of women have a textbook 28-day cycle.
The key hormones involved in this process are:
Follicle Stimulating Hormone (FSH): FSH is produced by a small gland in the brain called the pituitary gland. Think of FSH as the hormone that gets things started. Each month, it signals your ovaries to prepare several tiny sacs called follicles. Inside each follicle is an immature egg. FSH helps these follicles grow, but usually, only one will become mature enough to release an egg during ovulation.
Luteinizing Hormone (LH): LH is another hormone produced by the pituitary gland, but its role comes later in the cycle. After your follicles grow and mature, there’s a surge in LH, which triggers ovulation—the release of a mature egg from the ovary. This egg is now ready to be fertilised by sperm. LH also helps maintain the corpus luteum (a temporary structure formed from the follicle after ovulation) to support early pregnancy if fertilisation occurs.
Oestrogen: Oestrogen is primarily produced by the growing follicles in your ovaries. It’s responsible for several things: it helps your uterine lining (endometrium) grow thicker to prepare for a possible pregnancy, and it helps your body produce fertile cervical mucus, which makes it easier for sperm to travel through the reproductive system. Oestrogen is crucial for making sure your body is ready to support a pregnancy.
Progesterone: After ovulation, the structure left behind in the ovary (the corpus luteum) starts producing progesterone. Progesterone’s job is to prepare your uterus for a fertilised egg to implant. It thickens and stabilises the uterine lining, making it a comfortable environment for an embryo. If the egg isn’t fertilised, progesterone levels drop, and this causes the uterine lining to shed, leading to your period.
These hormones work together to guide your body through the four phases of the menstrual cycle.
The Four Phases of the Menstrual Cycle
1. Menstrual Phase (Days 1-5)
Your cycle begins with the first day of your period. During this phase:
The uterine lining (endometrium) sheds, resulting in menstrual flow. This shedding is triggered by a drop in oestrogen and progesterone levels at the end of the previous cycle.
Hormone levels are at their lowest. The decline in progesterone causes the blood vessels in the endometrium to constrict and then break, leading to tissue death and shedding.
The pituitary gland begins to increase its production of FSH, which will stimulate follicle growth in the next phase.
Fertility is typically low, but not impossible. While many women believe they can't get pregnant during their period, it's important to note that sperm can survive in the female reproductive tract for up to 5 days. If you have a shorter cycle, this could overlap with the next fertile phase.
The average blood loss during menstruation is about 30-40 millilitres, but anywhere from 5-80 mL is considered normal.
2. Follicular Phase (Days 6-14)
As menstruation ends, your body begins preparing for ovulation:
FSH levels rise, stimulating the growth of several ovarian follicles. Each follicle contains an immature egg (oocyte).
These growing follicles produce increasing amounts of oestrogen. This rise in oestrogen has several effects:
It causes the endometrium to thicken and become more vascularised, preparing for potential implantation.
It stimulates the production of fertile cervical mucus, which helps sperm survive and travel through the reproductive tract.
It triggers a negative feedback loop, causing the pituitary to reduce FSH production.
Usually, one dominant follicle continues to grow while the others are reabsorbed through a process called atresia. The dominant follicle can grow to be as large as 20-25 mm in diameter.
As oestrogen levels peak, they trigger a surge in LH production. This LH surge is crucial for the final maturation of the egg and the initiation of ovulation.
During this phase, fertility begins to increase. The length of this phase can vary significantly between women and even between cycles, which is why tracking ovulation is so important for those trying to conceive.
3. Ovulation Phase (Around Day 14)
Ovulation is the critical event for fertility:
The LH surge triggers several events in the dominant follicle:
The egg completes its first meiotic division, preparing it for potential fertilisation.
The follicle wall begins to break down, preparing to release the egg.
About 24-36 hours after the start of the LH surge, the mature egg is released from the ovary. This process is called ovulation.
The egg begins its journey down the fallopian tube, propelled by tiny hair-like structures called cilia.
Cervical mucus becomes thin, clear, and stretchy (often compared to raw egg whites), facilitating sperm movement through the reproductive tract.
The egg survives for about 12-24 hours after release, while sperm can live for up to 5 days in the female reproductive tract. This creates a fertile window of about 6 days centred around ovulation.
Signs of ovulation can include:
Changes in cervical mucus
A slight rise (0.2-0.4°C) in basal body temperature
Mild pelvic pain (mittelschmerz) in some women
Increased libido
Slight swelling of the vulva
4. Luteal Phase (Days 15-28)
After ovulation, the body prepares for potential implantation:
The ruptured follicle transforms into a structure called the corpus luteum. This transformation is crucial for fertility and early pregnancy support.
The corpus luteum, which means "yellow body" in Latin due to its yellowish appearance, is a temporary endocrine structure. It develops from the empty follicle left behind after ovulation.
The primary function of the corpus luteum is to produce progesterone, and to a lesser extent, oestrogen. Progesterone is essential for preparing the uterus for potential pregnancy.
Progesterone causes several changes in the uterus:
It stimulates the growth of blood vessels in the endometrium (uterine lining).
It causes the endometrium to become thicker and more spongy, ideal for embryo implantation.
It reduces the contractility of the uterine muscles, which helps maintain a potential pregnancy.
It stimulates the development of mammary glands in the breasts.
If fertilisation occurs, the developing embryo will produce human chorionic gonadotropin (hCG), which signals the corpus luteum to continue producing progesterone. This maintains the uterine lining and supports early pregnancy until the placenta takes over hormone production around 8-10 weeks of pregnancy.
If fertilisation doesn't occur, the corpus luteum will start to degenerate after about 14 days. This causes a drop in progesterone levels, which triggers the shedding of the uterine lining, marking the beginning of a new menstrual cycle.
The luteal phase is typically consistent, lasting about 14 days for most women. Understanding this phase is crucial for those trying to conceive, as a short luteal phase can sometimes make it difficult for a fertilised egg to implant successfully.
How Each Phase Affects Fertility
Understanding these phases is crucial for timing conception:
The "fertile window" includes the 5 days before ovulation and the day of ovulation. This is based on the lifespan of sperm (up to 5 days) and the egg (12-24 hours).
Having intercourse every other day during this window maximises chances of conception. This ensures a good supply of sperm is present when the egg is released.
After the egg is released, fertility declines quickly. The egg's short lifespan means that timing is crucial for natural conception.
The thickness and receptivity of the uterine lining, influenced by oestrogen and progesterone, also play a crucial role in successful implantation.
It's worth noting that stress, illness and other factors can delay ovulation, but the luteal phase usually remains constant. This is why women with longer cycles typically have a longer follicular phase, not a longer luteal phase.
Common Misconceptions About the Menstrual Cycle and Fertility
Let's debunk some myths:
Myth: Ovulation always occurs on day 14
Reality: Ovulation timing can vary greatly between women and cycles. For some women, it might occur as early as day 6 or as late as day 21.Myth: You can't get pregnant during your period
Reality: While unlikely, it's possible, especially for women with shorter cycles or longer periods. Sperm can survive long enough to fertilise an egg released soon after menstruation.Myth: Regular periods always mean regular ovulation
Reality: You can have regular periods without ovulating (anovulatory cycles). This is because the uterine lining can still build up and shed in response to hormonal fluctuations, even without an egg being released.Myth: Fertility apps can perfectly predict ovulation
Reality: While apps can be helpful, they often use averages and predictions. They may not account for individual variations in cycle length or timing of ovulation.
Factors That Can Affect the Menstrual Cycle
Several factors can influence your cycle:
Stress: High levels of stress can suppress GnRH (gonadotropin-releasing hormone), which can delay or prevent ovulation.
Diet and exercise: Extreme changes in weight or intensive exercise can disrupt hormonal balance. Both being significantly underweight or overweight can affect ovulation.
Medical conditions:
Polycystic Ovary Syndrome (PCOS): Can cause irregular periods and anovulation.
Endometriosis: Can cause painful periods and potentially affect fertility.
Thyroid disorders: Both hyperthyroidism and hypothyroidism can disrupt the menstrual cycle.
Age: As women approach menopause, cycles can become less predictable due to declining ovarian reserve and hormonal changes.
Medications: Some medications, including certain antidepressants and chemotherapy drugs, can affect the menstrual cycle.
Tracking Your Menstrual Cycle for Fertility
Tracking your cycle can provide valuable insights:
Calendar method: Simple but less accurate. It involves tracking the length of your cycles over several months to predict ovulation.
Basal Body Temperature (BBT): Your body temperature rises slightly (0.2-0.4°C) after ovulation due to increased progesterone. This method can confirm that ovulation has occurred but doesn't predict it in advance.
Ovulation Predictor Kits (OPKs): These detect the LH surge in urine, which occurs 24-36 hours before ovulation. They're more accurate in predicting ovulation than the calendar method.
Cervical Mucus Monitoring: As oestrogen levels rise before ovulation, cervical mucus becomes clear, slippery, and stretchy, resembling egg whites. This "fertile" mucus facilitates sperm transport.
Fertility Monitors: These devices track multiple signs of fertility, often including urinary hormone levels and BBT.
Many Australian women find apps helpful for tracking, but remember, technology is a tool, not a substitute for understanding your body.
The Role of the Menstrual Cycle in Fertility Treatments
In fertility treatments, we often manipulate the menstrual cycle:
Ovulation Induction: Medications like clomiphene citrate or letrozole are used to stimulate follicle development and ovulation in women who don't ovulate regularly.
Controlled Ovarian Hyperstimulation: Used in IVF, this involves using higher doses of FSH to stimulate the development of multiple follicles.
Luteal Phase Support: In some fertility treatments, progesterone supplements are given to support the endometrium and early pregnancy.
Cycle Suppression: In some IVF protocols, we use medications to suppress the natural cycle before stimulating follicle growth. This gives us more control over the timing of egg retrieval.
Understanding your natural cycle helps us tailor these treatments effectively and time procedures for the best chance of success.
Conclusion
The menstrual cycle is a remarkable process that holds the key to fertility. By understanding your cycle, you can better time conception attempts, identify potential issues, and work more effectively with your healthcare provider if you need assistance.
Remember, every woman's cycle is unique. What's normal for one person may not be normal for another. If you're concerned about your cycle or have been trying to conceive without success, don't hesitate to seek professional help. Fertility specialists are here to guide you through your journey to parenthood.
FAQ
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Yes, especially if you have a shorter cycle. Sperm can survive for up to 5 days in the female reproductive tract, so if ovulation occurs soon after your period, there's a chance of conception.
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The egg typically survives for about 12-24 hours after release from the ovary. This short lifespan is why timing is so crucial for conception.
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Cycle length itself doesn't necessarily affect fertility, but very irregular cycles might indicate hormonal imbalances that could impact fertility. Consistently short cycles (less than 21 days) or long cycles (more than 35 days) warrant investigation.
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As women age, cycles can become shorter and less regular due to declining ovarian reserve. Fertility also declines, particularly after age 35, due to both decreased egg quantity and quality.
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Yes, stress can delay ovulation or even cause anovulatory cycles by disrupting the hormonal balance necessary for ovulation. Managing stress through relaxation techniques or counselling can be beneficial when trying to conceive.
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If your cycles are consistently irregular (varying by more than 7-9 days), it's best to consult with a healthcare provider. They may recommend tests to check hormone levels or ultrasound to assess your ovaries and uterus.
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When used correctly, ovulation predictor kits are generally quite accurate in predicting ovulation 24-36 hours in advance. However, they may give false positives in some conditions like PCOS, where LH levels can be consistently elevated.
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While no specific food guarantees improved fertility, a balanced diet rich in fruits, vegetables, whole grains, and lean proteins supports overall reproductive health. Folic acid is particularly important for women trying to conceive. Some studies suggest that omega-3 fatty acids and antioxidants may be beneficial, but always consult with a healthcare provider before starting any new supplements.
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It can take a few months for your natural cycle to regulate after stopping hormonal birth control. Some women ovulate immediately, while others may take several months. During this time, cycles may be irregular as your body readjusts to its natural hormonal balance.
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For the most accurate results, wait until after your missed period. If you can't wait, test no earlier than 12-14 days after ovulation. Home pregnancy tests detect hCG in urine, which starts to be produced after implantation (about 6-12 days after ovulation). Testing too early can lead to false negatives.
Remember, this information is general in nature. For personalised advice, always consult with your healthcare provider or a fertility specialist.
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