A typical adult is many thousands of times larger than the size they were as a zygote, the fertilized egg cell that forms when sperm meets egg.

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.

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.

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.

Gestation is the period of time between conception and birth when a baby grows inside the mother's womb. Changes occur constantly over the course of these 40 weeks. At week 17, for instance, the skeleton of a fetus begins changing from soft...

As cells get larger, their volume grows much faster than the surface area through which nutrients and waste products pass, creating a bottleneck that eventually causes cells to die. This forces nearly all organisms visible to the naked eye to comprise enormous numbers of smaller cells, such as the approximately 30 trillion found in the average adult human.

Embryonic stem cells emerge around five days after fertilization, during the blastocyst stage of embryonic development. At this point, the embryo consists of about 100 to 150 cells and forms a hollow structure, and these cells are the ultimate source of all other cell types during fetal development.

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.

In this series of comic-style panels, a "fellowship" of sperm cells travels through fictionalized versions of reproductive anatomy on their way to an egg cell. Each panel includes informative commentary on each step of the adventure, with research insights provided by a reproductive biologist, a biomedical scientist, and an andrologist.

As part of the cell cycle, this ensures that after mitosis—the process of cell division—the two resulting daughter cells possess the tools necessary to complete the functions of their parent cell, including accumulating energy and making proteins. During meiosis, four egg or sperm cells are created, each with half the parent cell's DNA.

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.

Girls are born with an estimated 1 million to 2 million eggs in their bodies, but around 10,000 die monthly before girls hit puberty. Scientists have not yet figured out why there's such an excessive number of eggs prior to menstruation, but they've calculated that there are roughly 300,000 to 400,000 eggs left at puberty.

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 first weeks of embryonic development, known as embryogenesis, are one of the most critical phases during pregnancy. This simple breakdown from the Khan Academy illustrates what we know about how the complex process unfolds.

Scientists observed zebrafish embryos as they grew and isolated the several-second-long initial heartbeats to better understand this crucial moment in every heart's life. The zebrafish's heart starts beating 20 hours into their existence. Each heart cell activates electrically at random, without rhythm. As these cells mature, they become more sensitive to electrical charges; as they all fire, a rhythm is established and a pulse is born.

Humans have over twice as long childhoods than other animals with similar lifespans like whales and elephants. A human child can grow for nearly 20 years before they become a reproducing adult. Researchers believe there's a connection between long childhoods and human's unusually large brain size, though the science is not totally clear. Dive into what scientists know here.

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.

The main difference between embryonic and adult stem cells lies in their potential and origin. The former can become any cell type in the body, while the latter are limited to specific cell types. Embryonic stem cells also carry ethical concerns when used in medical or clinical applications.

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.

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.

Genetic chimeras can result when cells with different DNA combine, such as when two sperm cells fertilize two egg cells, creating two embryos that then merge. In mosaicism, mutations arise within an organism after conception and can spread through cell division, resulting in diseases and forming genetically distinct anatomy.

While birthing twins has been a rare occurrence for humans, research suggests that twins were actually the norm much further back in primate evolution. It’s proposed that single-childbearing turned out to be critical for the evolution of larger babies with larger heads. Discover more about the shift that gives insight into primate evolution.

In humans, the 2-meter (6.56-foot) molecule wraps around spool-like proteins called histones, forming beads—nucleosomes—that fold up into fibers. These fibers—chromatin—form loops through folding that compress into larger fibers, which tightly coil together to form a chromosome.

While fish, amphibians and reptiles get bigger throughout their lives until they die, their slower metabolisms allow their bodies to regulate the resulting smaller temperature increases. In humans, hormones triggered during puberty harden growth plates, ending bone lengthening.

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.

The insects' fast life cycle and affordability have allowed geneticists to quickly identify which genes are responsible for limbs, eyes, and other body structures by inducing mutations. The same genes may be responsible for different parts of a species' body, such as limb-building genes being responsible for wing pigmentation in butterflies.

Humans have had a long history. 3.8 billion years in the making, to be precise. From the primordial puddle to the modern day, here's how humans have evolved from the first life.

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.

The human brain, in all its staggering complexity, is the product of millions of years of evolution. Read on to find out about brain size and intelligence, language and brain development, and more.

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.

Science has come to understand different types of aging. What we call age is simply the number of years since birth, or chronological age. At the cellular level, biological aging happens at different rates for different people, and explains why some seem to age more quickly (or slowly).

The human genome contains over six billion DNA letters, organized into 46 chromosomes inherited from both parents. Genes—sections of DNA—serve as templates for making proteins, and variations in these sequences can affect how our bodies work or respond to the environment.

In ionic bonding, one atom transfers at least one electron to another atom, forming two atoms of opposite charge. These atoms attract one another and can stack up in a 3D grid called a lattice. Covalent bonding, where electrons are instead shared, can produce enormous molecules, such as the two found in human chromosome 13, which each contain 37 billion atoms.

There are many more genes on Earth today than when microbial life first spawned on the planet about four billion years ago. Many of these newer genes grew from genetic mutations, misprints that slowly drift from the original blueprint via cellular division. If these mutations drift far enough, they can establish a new family of genes, though this process can take well over a million years.

Cells are the building blocks of life, forming tissues and organs that keep our bodies working. As we age, cellular changes have progressive consequences on both the structure and function of our organs. This article examines this complex relationship as it relates to the incremental changes associated with aging.

Chromosomal irregularities are more likely at older maternal ages so genetic testing for a fetus of mothers-to-be at age 35 and older is often recommended. Conditions that can be screened for in utero include cystic fibrosis, Tay-Sachs disease, sickle cell disease, Down syndrome, spina bifida, Duchenne muscular dystrophy, and more.

Stem cells are different from other cells because they are capable of creating copies of themselves and producing other types of cells. For example, the ones found in our brains, bones, or muscles. Curious about stem cells? Here's a breakdown of how they work.