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Dear residents, ovulation is something we all study in textbooks, but when it comes to understanding how beautifully the female body works, most books fall short. Ovulation is not just about an egg being released every month; it is a journey that starts even before a girl is born. 

In this blog, we walk through ovulation and menstrual physiology the same way Dr. Japleen Kaur explains it, step by step, logically, and with clear clinical relevance. 

The Journey of an Egg Begins Before Birth 

Most people don’t realise that a woman is born with all the eggs she will ever have. 

During early fetal life, special cells called oogonia travel from the yolk sac to the developing ovaries. By the time a baby girl is 20 weeks old inside her mother’s womb, she already has nearly 7 million eggs. 

After that, nature slowly starts reducing this number. 

• At birth, only about 1 million eggs remain 

• By puberty, the number drops to around 2 lakh 

• By the age of 30, only about 26,000 eggs are left 

Out of all these, only about 400 eggs will ever be released in a woman’s lifetime. The rest slowly disappear — a natural process called atresia. 

What Happens to the Egg Before Ovulation? 

From birth till puberty, all eggs stay in a resting stage. They are paused in Meiosis I, waiting for the right time. 

When ovulation happens, the chosen egg wakes up and continues dividing: 

• It completes its first division 

• Releases the first polar body 

• Becomes a secondary oocyte 

• Then pauses again in Meiosis II 

Only after fertilisation does the final division take place and a mature ovum is formed. 

Why Oocyte Maturity Matters in IVF? 

In IVF treatment, doctors want to collect only fully mature eggs. 

An immature egg cannot be fertilised properly. A mature egg (called an M2 oocyte) has already completed its first division and is ready for fertilisation. That’s why embryologists carefully examine every egg under the microscope before proceeding. 

Any error during this stage can lead to genetic problems, which is why this step is extremely important. 

Maternal Age and Chromosomal Problems 

One very important clinical fact is the relationship between maternal age and chromosomal disorders. 

Among all chromosomal abnormalities, Down syndrome (Trisomy 21) is the one that clearly increases as maternal age increases. This is why, when counselling older pregnant women, doctors focus mainly on the risk of Down syndrome. 

How Hormones Control Ovulation?

Ovulation is controlled by a beautiful hormonal chain reaction. 

The hypothalamus in the brain releases GnRH in small pulses. This stimulates the pituitary gland to release FSH and LH. 

• FSH acts on the granulosa cells of the ovary 

• LH acts on the theca cells 

Theca cells produce androgens, which are converted into estrogen inside granulosa cells. 

After ovulation, the same hormones help produce progesterone, which prepares the uterus for pregnancy. 

The Feedback System That Keeps Everything in Balance 

The menstrual cycle stays regular because of a smart feedback system. 

• Estrogen tells the brain when enough hormone has been produced 

• Progesterone tells the brain when ovulation has already happened 

These hormones switch off further hormone production at the right time so that the cycle remains balanced. 

How a Follicle Grows Inside the Ovary?

Every month, several tiny follicles start growing inside the ovary. 

1. Primordial follicle – a resting egg surrounded by a few cells 

2. Primary follicle – the egg grows and forms a protective layer 

3. Secondary follicle – a fluid-filled cavity appears (this is what we see on ultrasound) 

4. Mature follicle – grows up to about 20 mm and is ready to release the egg 

Ovulation and Formation of Corpus Luteum 

When ovulation occurs, the mature follicle ruptures and releases the egg. The remaining follicle transforms into the corpus luteum, which produces progesterone and supports early pregnancy. 

On ultrasound, it appears like a small hemorrhagic structure with blood flow around it. 

Why Only One Egg Is Released Each Month?

Although many follicles start growing, only one usually wins the race. 

This happens because the winning follicle responds best to FSH. It produces more estrogen, which lowers FSH levels and stops the other follicles from growing. The rest slowly shrink and disappear. 

How does IVF Changes This Natural Process? 

In IVF, doctors give FSH injections from outside. This keeps FSH levels high for longer and allows multiple follicles to grow together. That’s how several eggs can be collected in one cycle. 

Role of Ovulation Induction Medicines 

Two common medicines are used to help women ovulate: 

Clomiphene citrate tricks the brain into thinking estrogen levels are low, so more FSH is released. 

Letrozole reduces estrogen production, again increasing FSH levels. 

Both help trigger ovulation in women who are not ovulating naturally. 

Conclusion: 

Ovulation is one of the most fascinating processes in the human body. It is controlled by hormones, shaped by genetics, and guided by a perfect internal clock. 

From the time an egg is formed in fetal life to the moment it is released during ovulation, every step has clinical importance, especially in fertility treatment and reproductive medicine. 

Understanding this process properly makes you a better clinician and a more confident OBG resident.