The Complex Journey to an Embryo

Each and every one of us, like most multicellular organisms, began life as a zygote—a fertilized egg containing genetic material from both the female and the male. However, for that single cell to develop into a complex and independent being, two intricate processes are required—fertilization and pregnancy—which occur within the female reproductive system. To better understand these processes, we must focus on this system, composed of several organs working together, including the vagina, uterus, cervix, fallopian tubes, and ovaries.

Developing from a single cell into a complex being requires two intricate processes—fertilization and pregnancy—which occur in the female reproductive system | Shutterstock, Life science

Where Have My Eggs Gone?

While men begin producing sperm cells only at puberty, every woman is born with a fixed number of gametes—eggs—that decrease gradually over her lifetime. A female fetus has about six million eggs, a number that drops to one to two million by the time she is born. By the time she reaches sexual maturity, when the reproductive system becomes fully functional, only about 350,000 eggs remain. Once she reaches adulthood, approximately 1,000 eggs are lost each month, with only one undergoing ovulation. Ovulation ceases on average around the age of fifty, during the phase known as menopause, marking the end of natural reproductive capability.

The ovaries are a pair of oval-shaped glands located on either side of the uterus. They produce, store, and release eggs according to the menstrual cycle, which typically lasts an average of 28 days. The female sex hormones, estrogen and progesterone, are also produced in the ovaries and play vital roles in numerous processes, including preparing the uterus for implantation of a fertilized egg and pregnancy, regulating the menstrual cycle, developing mammary glands in the breasts, and balancing blood sugar and cholesterol levels. These hormones also influence areas beyond reproduction, such as memory and mood.

Female sex hormones participate in numerous critical processes. Hormone levels, egg maturation, and uterine lining changes throughout the menstrual cycle | Shutterstock, Dee-sign

The Chosen Follicle

Egg cells are housed within tiny structures called follicles, and their maturation and ovulation involve a complex regulatory process. At puberty, the hypothalamus—an area in the brain functioning as a "control center"—begins secreting the hormone GnRH. This hormone is released into the bloodstream and reaches the anterior pituitary gland, which produces the hormones essential for ovulation: FSH (follicle-stimulating hormone) and LH (luteinizing hormone). These hormones travel through the bloodstream to the ovaries, where they trigger and regulate follicle development and ovulation.

Each month, under the influence of LH and FSH, five to fifteen follicles start maturing. Only one of them will complete the maturation process and become the "dominant follicle," primarily because it has the most receptors for FSH. The remaining follicles will degenerate and die. The dominant follicle undergoes advanced maturation processes, allowing the egg inside it to fully develop. Midway through the menstrual cycle, the dominant follicle ruptures, releasing the mature egg, which continues its journey through the reproductive system.

The Journey to the Uterus

Thus, each month, from puberty to the end of ovulation, an egg is released from one of the ovaries and immediately enters the fallopian tube. The fallopian tubes are two muscular tubes located on either side of the uterus. The egg travels through one of these tubes on its way to the uterus. At the end of the fallopian tube, the fimbriae—fringe-like structures resembling a hand—coordinate with the ovary to sweep the egg into the tube. Inside the tube, the egg moves with the help of muscular contractions and relaxations, as well as the motion of cilia, hair-like extracellular structures lining the tube’s interior. Together, these mechanisms enable the egg to travel toward the uterus.

The fimbriae coordinate with the ovary to sweep the egg into the fallopian tube | Shutterstock, grayjay

The egg travels along the fallopian tube for about 30 hours. In most cases, it remains unfertilized, degenerates, and is expelled from the body with menstrual blood, ending its journey. However, an egg that encounters a sperm cell along the way will reach the uterus fertilized and implant there for approximately forty weeks (280 days). During this period, the fertilized egg undergoes countless cell divisions, gradually developing from a cluster of cells into organs and systems, eventually forming a complete fetus. While in the uterus, the fetus receives all its needs from the mother through the placenta and umbilical cord until it is ready to enter the world.

A Long and Winding Journey

In natural fertilization, sperm cells undertake a convoluted path to reach their destination. During sexual intercourse, millions of sperm cells are released—often more than 200 million. Each sperm cell travels from the male reproductive system into the vagina, a flexible canal connecting the body’s exterior to the cervix. During childbirth, the vagina becomes the birth canal, and it also serves as the route for menstrual blood to exit the body.

Semen is produced in the male reproductive system and contains sperm cells alongside substances that assist them: sugar for energy, lipids to improve their movement and stimulate uterine muscle contractions, bicarbonate to protect against vaginal acidity, and more. Semen is deposited into the vagina, and the sperm begins its journey through the cervix. The rest depends on the cervix's condition: for most of the menstrual cycle, the cervix is sealed with a thick mucus layer that blocks sperm and other potential contaminants from entering the uterus. During ovulation, when the egg is ready for fertilization, the mucus becomes more fluid, allowing sperm to pass. In such cases, sperm travel from the vagina through the cervix into the uterus and, if successful, meet the egg in the fallopian tube. Sperm primarily move using their "tail," a structure called a flagellum, which propels the cell forward.

The sperm travels from the vagina to the cervix and the uterus, ultimately meeting the egg in the fallopian tube if successful. Illustration of the female reproductive system | Shutterstock, Lightspring

The Few That Survive

Tens of millions of sperm begin the journey, but only about 200 make it to the fallopian tubes, all with one goal: to penetrate the egg’s outer layer. The first sperm reach the fallopian tube just minutes after entering the vagina and can survive in the female reproductive system for up to five days, increasing the chances of fertilization.

The most critical moment occurs when the sperm meets the egg. This typically happens in the fallopian tube, usually in its middle, wider section called the ampulla. Fertilization begins when a single sperm penetrates the egg’s outer layers and transfers its genetic material inside. This triggers a cascade of chemical reactions in the egg, including a surge of calcium and zinc ions and the hardening of the egg’s membrane, preventing additional sperm from entering.

The next significant stage in the fertilized egg's life begins about 24 hours after fertilization. The egg starts cell division—from one cell to two, from two to four, and so on. This division occurs as the fertilized egg moves along the fallopian tube toward the uterus. After five to six days, the developing embryo consists of about 100 cells and is referred to as a blastocyst. The blastocyst reaches the uterus, where it implants into the uterine lining and continues its development.

Cell division occurs as the fertilized egg moves along the fallopian tube toward the uterus. Illustration of the early divisions of the egg | Shutterstock, DAntes Design

There's No Place Like Home

The uterus is the final destination of the fertilized egg and one of the most important organs in the female reproductive system. It is where the fertilized egg implants, and where the embryo receives nourishment, protection, and suitable environmental conditions for development until birth. The uterus is pear-shaped, located in the pelvis, and composed of three layers: the endometrium, the inner layer where the embryo implants if fertilization occurs, and which disintegrates and is expelled during menstruation if it does not; the myometrium, the muscular middle layer that allows the uterus to expand during pregnancy; and the perimetrium, the thin, slippery outer layer that facilitates the movement of the uterus within the pelvic and abdominal cavities.

From the beginning of ovulation, the uterus undergoes changes to prepare for the fertilized egg. The primary change is the thickening of the endometrium and increased blood supply to it. This creates an environment suitable for the fertilized egg's implantation and, subsequently, for the embryo's development and nourishment. During pregnancy, the uterus increases in volume by 500 to 2,000 times its pre-pregnancy size—from 10 milliliters to an average of five liters, and sometimes even up to 20 liters.

The blastocyst, developed from the fertilized egg, reaches the uterus on average seven days after fertilization. At this stage, it attempts to attach to the endometrium and implant into the uterine wall to begin developing and receiving nourishment from the mother. If implantation fails, the blastocyst is expelled from the body along with the endometrium, and the woman experiences a regular menstrual period without knowing that fertilization occurred. Only about half of fertilized eggs successfully implant; the rest are naturally expelled from the body.

The blastocyst continues its development into an embryo, with its age corresponding to the gestational age. Gestational age is typically counted in days and weeks from the date of the mother’s last menstrual period and verified through ultrasound, which is considered more accurate. Accurately determining gestational age is essential for assessing the embryo’s growth and development, scheduling routine tests, and providing the mother with necessary medications and vaccinations. Therefore, following medical recommendations for monitoring is crucial for safeguarding the health of both the mother and the baby.

Accurate determination of gestational age is essential for assessing the embryo’s growth and development. Illustration of embryo development | Shutterstock, YUCALORA

Infertility

Infertility is defined as the inability to achieve pregnancy after a year or more of unprotected sexual intercourse during ovulation. The prevalence of infertility is approximately 15%, meaning that one in seven couples will not conceive naturally within a year. Fertilization is a complex process that requires precise synchronization of multiple factors, including the timing and levels of hormone secretion, proper functioning of reproductive organs, and successful implantation of the fertilized egg. Disruptions in any of these processes reduce the likelihood of pregnancy.

In the past, infertility was often attributed solely to the female partner, but research has shown that both partners contribute equally to infertility, and in some cases, both are responsible. Couples who have not conceived naturally after a year are advised to seek medical consultation to identify the underlying cause and improve their chances of conception.

The primary cause of infertility related to the female reproductive system is irregular ovulation. The most common factors contributing to this are age, due to a decline in both the number and quality of eggs, and hormonal imbalances, which can disrupt ovulation. Other factors include blocked fallopian tubes, which prevent sperm and eggs from meeting, and uterine abnormalities, such as congenital defects or tumors, which can hinder implantation and development of the fertilized egg. To diagnose the issue, women may undergo blood tests and additional examinations, such as imaging and pelvic scans.

Infertility originating in the male reproductive system often manifests as a reduction in the quantity or quality of semen. Key aspects assessed include sperm morphology, motility, concentration, as well as the pH level, volume, and viscosity of the semen. Despite comprehensive testing and advanced technology, the cause of male infertility remains unknown in approximately half of the cases. The most common cause of reduced male fertility is damage to the testicles, where sperm is produced. Conditions such as varicose veins in the testicles, inflammation, or the effects of medications and surgeries can reduce sperm quality. Other causes include testosterone deficiency, congenital abnormalities such as blockages or absence of the sperm duct, and undescended testicles (cryptorchidism).

One in seven couples will not conceive naturally within a year. Negative pregnancy test | Shutterstock, Taimit

So, What Can Be Done?

Fertility treatments provide a solution for a significant portion of infertility cases. In the United States, 42% of the adult population report either undergoing fertility treatments themselves or knowing someone who has. There are various types of fertility treatments, each tailored to address the specific issue faced by the couple in question.

When infertility originates in the male reproductive system, several solutions are available. If the vas deferens (sperm duct) is blocked and sperm cannot be released, surgical extraction of sperm directly from the testicle can be performed to fertilize eggs. In cases of low sperm concentration due to hormonal deficiencies, the man can be prescribed the necessary hormones as medication. If a testicular infection is causing low sperm count, antibiotic treatment may help. Sometimes, conservative measures such as regular exercise, weight loss, or quitting smoking and alcohol consumption can improve fertility.

When infertility is linked to the female reproductive system, treatment often involves ovulation induction using hormones, egg retrieval and fertilization outside the body (in vitro fertilization), or surgery to correct or bypass structural abnormalities causing infertility.

All that remains is to wish all involved success in their treatments.