View an illustrative journey of sperm cells navigating toward fertilization

Understand the complex, biological journey of pregnancy with this science explainer.

Scientists first observed a sperm and an egg uniting in 1876

Throughout human history, scholars and researchers have speculated on the origin of human reproduction. It wasn't until the late-19th century that the physical process of egg fertilization via sperm cells was observed (in sea urchins), forming the basis for modern fertility research.

Fruit fly sperm are gigantic—about 40 times larger than human sperm, according to scientific research published in June 2026.

Only about 0.0001% of sperm, on average, successfully reach the uterine cavity

We've evolved to make pregnancy a complex and difficult process. From the initial development of the blastocyst, to the energy and nutrients needed to grow a new human, our bodies are both fighting and supporting fetal development.

Examine this captivating animation 'building' a human—illustrating the roughly 40 weeks of fetal development.

Explore this interactive visualization of the IVF journey as a parent or child

Follow an IVF procedure from lab to transplant

What happens once eggs are harvested from an ovary? How does an embryo form within a petri dish? This detailed video offers a glimpse of the transformations eggs experience as women age, and how sperm are injected into eggs in some forms of IVF treatment.

A new AI-assisted reproductive technique is finding 'hidden' sperm cells for those with very low sperm counts

Known as the Star system—for Sperm Track and Recovery—it uses artificial intelligence to help identify and locate "hidden" sperm in men with a rare infertility condition called azoospermia. The condition affects about 1% of men, and these individuals have sperm counts low enough that the individual spermatozoa in their ejaculate may be undetectable. In late 2025, the first baby was born after the child's parents used this assistive technique to locate sperm.

A step-by-step overview of the IVF process

In-vitro fertilization involves a sequence of complex steps to move from egg production to birth. This include stimulating egg production in the ovaries, egg harvesting and fertilization outside the body, embryo culturing and transfer, and methods to support in and increase the likelihood of full-term pregnancy.

Scientific advances are unlocking a wide range of alternatives to IVF

From refined artificial insemination and microsurgical sperm extraction to experimental uterine transplants, scientific innovation is rapidly advancing potential fertilization technologies. But they come along with costly trials, some ethical dilemmas, regulatory issues, and more.

A gallery of medical illustrations from NIH

The National Institutes of Health provides educational materials for broad use. The institute hosts a Flickr account with many different image galleries, including this one (with over 70 images to date) that illustrate and annotate curious phenomena, from the life cycles of a malaria parasite to the comparison of male and female bladders. Explore the gallery here.

The number and quality of sperm in men's semen is down, giving rise to various movements to avoid 'spermpocalypse'

A typical fertile, healthy young man produces over 100 million spermatozoa, or sperm cells, in his testicles every day. Scientists agree sperm figures are generally moving in the wrong direction, but also emphasize there's still a lot to learn about sperm.

Watch a salamander grow from fertilization to hatching

New microscopy techniques have allowed scientists to image the development of animal embryos with unprecedented detail. This six-minute time-lapse reveals the development of a salamander from initial fertilization to hatching from an egg.

Explore this week-by-week visualization of pregnancy

This graphic illustrates how development proceeds during each week of pregnancy, tracking what shifts and milestones typically occur during each stage of the process. It also includes typical weights of the fetus throughout.

In vitro gametogenesis: using induced stem cells to fertilize eggs

Using stem cells from skin and mouth cells could save hopeful parents time and money and spare egg donors from significant amounts of discomfort throughout the process. There are potential consequences, not just for infertility treatment, but for the ability to create customized cells of any type.

Various pollutants may be contributing to a male infertility crisis

Male infertility, due to declining sperm counts or quality, accounts for about half of the cases of infertility among couples. Recent research has found that global sperm counts fell, on average, by 1.2% per year between 1973 and 2018—from 104 million to 49 million per milliliter. From 2000, this rate of decline accelerated to more than 2.6% per year. (Some users may encounter a paywall.)

AI can aid fertility treatments, from helping select egg candidates to laboratory automation

Reproductive AI technologies are already here, from processing vast amounts of data to helping clinicians identify the healthiest embryos for implantation. It could be a boon to the fertility industry, and help mitigate the shortage of experienced embryologists.

Explore what each organelle does within various types of cells

In "Cell Anatomy Viewer," users can explore interactive illustrations of animal, plant, fungal, and bacterial cells. Clicking on an organelle, such as a lysosome, the nucleolus, or the Golgi complex, provides descriptions of its characteristics and functions.

The human body in 3D

Zygote Body is a free human anatomy tool that allows users to explore the intricacies of the human body in stunning detail. With the click of a mouse, users can isolate by body system or view anatomical structures from any angle. Check out this...

The secret lives of cells

Cutting-edge microscopy has revealed previously unseen interactions of biological molecules within their cellular environment, enabling scientists to observe and analyze the molecules in a holistic view. This podcast explains the minute advancements made over the last few decades in observing cells, revealing how scientists have come and what the future holds for the field.

IVF procedures fertilize eggs outside the body in a laboratory dish

In vitro fertilization is a complex and expensive set of procedures that potential parents dealing with infertility or genetic disorders may turn to when other methods are unavailable. IVF births account for 2% to 3% of all live births in the US annually.

'Artificial embryos': The hidden steps in forming a spine

Somitogenesis is the process by which segmented body structures like vertebrae form in embryos. While the process is well understood in animals like mice or zebrafish it is difficult to study in humans. But now, researchers have created a model of somitogenesis, using pluripotent stem cells, called an axioloid.

An interactive guide to the cardiovascular system

The cardiovascular or circulatory system moves blood throughout the body, ensuring cells have necessary nutrients and removing any waste. Click through this interactive 3D guide to explore the system as it crisscrosses the body via arteries, veins, and capillaries. This guide also includes similar breakdowns of the body and its other processes.

View a gallery of images and videos of over 200 types of cells

Funded by the National Institute of General Medical Sciences of the National Institutes of Health, the Cell Image Library houses over 60,000 pieces of media, including microscopic photographs, time-lapse videos of cellular activity, and 3D reconstructions of organic processes.

Take a 360-degree look at biochemical activity from within a human cell

In this tour, users can observe the dynamic processes that occur within the basic units of the human body, including the production and transport of proteins and the assembly and disassembly of microtubules—thin, cylindrical structures that provide structural support and a means of moving material within the cell.

A visual database of pollens

Pollen is a powdery substance made up of small grains involved in the fertilization process of plants. There are thousands of types of pollens across the world, and you can see real images of them in this visual database.

Exploring the link between history, art, and anatomy

Throughout centuries, art has played a vital role in sharing—and furthering—scientific understanding. This largely began in 1543 when physician Andreas Vesalius worked with an artist and engraver to transform learnings from his dissections into...

Infertility is often not discussed, forcing women and families to grieve privately

Watch this podcast discussion from Weill Cornell Medicine about the emotional journey of infertility for couples and individuals and hear advice about how to navigate the feelings of loss of control, grief, anger, and despair, including suggested tips for supporting individuals struggling with infertility.

What is a cell? A great quick overview.

YourGenome.org furnishes a collection of information on cells and how they work. Every body has trillions of cells, each serving a specific function: making proteins, processing chemicals, and creating energy. Every cell contains a genome, a set of instructions for the cell's function. Brush up on your cells for a better grasp on genetics.

One dad, one hundred babies

A look at the companies offering surrogacy to the ultrarich—including Chinese men taking their reproductive cues from Elon Musk.

Sex determination isn't all XX and XY

We're generally taught that chromosomes determine an animal's sex, but it is way more nuanced than that. Watch this 12-minute video to find out how.

'Cells Making Cells': 'Science Sam' discusses the potential of stem cells

In this episode of the Ologies podcast, host Alie Ward picks the brain of science communicator Samantha Yammine—whose PhD work focused on neural stem cells. The conversation covers everything from the earliest days of stem cell research to what the future of stem cell therapies may look like.

How the body produces urine

Things seem so simple when nature calls. But, how does the body actually produce the urine we so regularly and thoughtlessly expel? This detailed, comprehensive animation walks viewers through every step with precise illustrations, from the kidney's filters (known as nephrons) to the direct nerve connecting the bladder to the brain. See how the body does it here.

The love lives of pygmy seahorses

Inch-long pygmy seahorses live fascinating lives in the Coral Triangle of Southeast Asia, where the tiny sea creatures thrive. They are one of the few species in the world where the male carries its progeny to term, in this case, after receiving an egg from the female, who keeps busy checking in on her two male mates, one of whom is always pregnant. Dive into the marine romance in this fascinating essay.

The inner life of a cell

White blood cells are responsible for protecting the human body from infection. This video uses biophysical data from Harvard researchers to accurately demonstrate what goes on inside one white blood cell as it works within the immune system.

Stem cells become specialized cells through the activation or silencing of specific genes

After fertilization, when a sperm cell fuses with an egg, the resulting stem cell is described as totipotent—capable of transforming into any type of body cell or embryonic structures (e.g., placenta). Once differentiated into multipotent cells, they can only specialize further within specific cell type families, such as hematopoietic cells that become platelets and red and white blood cells.

The history of ectogenesis and artificial wombs

While the science of artificial wombs is new, the idea goes back to a 1923 Heretics Society of the University of Cambridge lecture where an English biologist was one of the first people to propose an egg could be fertilized outside the womb. This...

How two families navigated an unthinkable IVF mistake

The field offers hope to thousands of families annually, but it is also largely unregulated, and for a small number of parents, things can go terribly wrong.

The cells of spotted salamander embryos and early-stage larvae harbor algae that support embryonic development by increasing oxygen levels and removing waste.

Explore the human family tree

This interactive human family tree shows the various species that constitute the human evolutionary family. Click on each species or group image to learn more.

Watch a tour of an integrated circuit fabrication plant

Explore the protocols in place to ensure clean rooms have no dust to jeopardize the integrity of silicon wafers as they are transformed into working microchips. Layering, photolithography, etching, and chemical vapor deposition are shown alongside digital visualizations.

Find fertility clinics and their success rates in your state

This database from the Centers for Disease Control and Prevention provides a searchable portal of fertility clinics across the US, with reported data on services, total procedures performed, and live births recorded for as recently as 2022.

Pollinators help develop seeds by transferring pollen between flowering plants

As bees approach a bright, colorful flower for nectar, they bump into its stamen—the flower's male reproductive organ—and pollen gets stuck to the pollinator's body. This pollen can then be deposited on another flower's sticky pistil—the flower's female reproductive organ—which brings the pollen down to the ovule for fertilization.

Explore how the placenta acts as lungs, kidneys, and more in the womb.

Visualize the anatomy of a comet

The solid nucleus holds the frozen molecules, which are heated into a gaseous state once the comet is within three times Earth's distance from the sun. The surrounding coma, from which comets get their name, is a spherical cloud that can grow larger than Jupiter, from which tails develop.

An interactive dive into the immune system

Our bodies are equipped with an incredible system able to fight off invasive pathogens to protect us from illness. The complicated system includes lymph nodes, the spleen, bone marrow, and even the tonsils. But this same system can turn against itself, a condition known as autoimmune disorders. Follow along with this thorough, engaging explainer on the human body's defense system.

Explore how cells respond to signals from their environment

In "Dropping Signals," users can drag stimuli, such as hormones and light, into various cellular environments to observe cellular responses. These include changing their shape, enabling the transport of chemicals into or out of the cell via the cell membrane, or dividing to compensate for surrounding damaged cells.

Explore a series of interdisciplinary explainers on climate science

From animations on the quantum mechanics of greenhouse gases to a photo essay on how scientists acquire climate data, this collection provides detailed explorations of how different branches of science have contributed to our understanding of climatology.

Interactive human migration map

Using archeological, genetic, and climate data, scientists have pieced together an outline of the human odyssey—the journey that took our species from Africa to all corners of the globe. Experience this 200,000-year migration using an interactive human migration map.

3-parent baby approach may help women with infertility

Though the topline promise of this approach is avoiding passing on mitochondrial diseases, some researchers and companies have been eyeing its potential for helping with female infertility, since faculty mitochondria may contribute to the condition. Recent research suggests this approach could be promising.