Fetal development, week by week

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.

What happens during the first eight weeks of pregnancy?

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.

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

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

In the womb, a fetus may begin to hear at 18 weeks

Though little indentations form on the side of the fetus's neck at roughly nine weeks—with some formations taking place even before then—developmentally, these indentations don't become ears that allow a little one to hear until around 18 weeks.

Touch is the first sense a fetus develops in the womb

During gestation, our sense of touch begins to develop around 8 weeks, starting with the development of sensory receptors in the face and, about a month later, in the palms and soles of the feet.

A baby's due date is calculated from the first day of the last menstrual period—not when sexual intercourse occurred.

Babies taste food in the womb via amniotic fluid, and it may influence future taste buds.

Every organ in a mother's body changes during pregnancy as her body produces hormones that relax muscles, allowing for the needed expansion.

Pregnancy causes a woman's blood volume to increase by up to 50%.

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

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.

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.

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.

Each day, about 60 US pregnancies end in stillbirth

This online memorial for families who experienced the unexpected death of a fetus 20 weeks or older is the result of ProPublica reporters inviting 60 families to share their baby's name and answer some questions about their unfulfilled wishes for their children. About 21,000 babies are stillborn in the US annually. About the same number of children die in the US during their first year of life.

A pregnant woman's brain sheds gray matter, perhaps explaining 'baby brain.'

Your feet can expand—permanently—due to pregnancy.

Pregnancy at older ages increases the chances of fetal abnormalities

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.

The syncytiotrophoblast is the outer layer of the placenta and the largest human cell, spanning an area about twice that of a king-sized bedsheet.

Gestational diabetes occurs when the body can't produce insulin during pregnancy

Globally, pregnant people are increasingly at risk to develop gestational diabetes due to rising maternal ages and obesity rates. The condition is associated with heavier fetal weight, congenital abnormalities, and perinatal mortality, which includes stillbirths and deaths of babies younger than 7 days old.

Artificial wombs and preterm births: an overview

Scientists are working to create artificial wombs that would help preterm babies survive their early entry into the world. But how much help could this technology provide, and how much of a problem are preterm births? This article details some of...

The science and ethics of artificial wombs

Artificial wombs may be able to help preterm babies survive their early entry into the world and possibly curb the long-term adverse health effects associated with such premature birth. This article dives into the current ethics and science behind artificial wombs and ectogenesis, which is when a fetus develops in artificial conditions.

Embryonic stem cells can develop into any other cell type in the body

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.

Heightened sense of smell during pregnancy is not yet well understood

Boosted smell powers and strong reactions to them are often reported beginning around the six-week mark of pregnancy. Researchers say hormonal changes likely play a role, but the change remains poorly understood.

'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.

Chagas disease can be transmitted to a fetus during pregnancy

Researchers estimate that 63 to 315 cases of congenital Chagas disease occur annually in the US. Roughly 20 to 30 percent of all Chagas disease cases will progress to cardiac or gastrointestinal disease.

Modern pregnancy tests detect a specific hormone in a woman's urine or blood.

Using artificial wombs to improve preterm infant health outcomes

Researchers at a hospital in the US have successfully used artificial wombs to care for preterm lambs and are now looking at the possibility of human trials. This nine-minute video includes interviews with the researchers and explains the reasoning...

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.

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...

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

How the heart achieves its first beat

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.

View an illustrative journey of sperm cells navigating toward fertilization

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.

Exposing fetuses to certain flavors in the womb could lead to children being less picky eaters

A new study found that 3-year-olds were less averse to the smell of vegetables they had been repeatedly exposed to in utero, suggesting that what mothers eat in late pregnancy can leave a lasting imprint on their children's tastes.

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.

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...

Hair loss after childbirth happens because pregnancy hormones paused the normal hair shedding process during pregnancy, so the hair loss is actually your hair cycle 'catching up' with its normal process several months postpartum.

In 1984, a dying infant lived for three weeks with a baboon heart inside her chest.

The ancient Egyptians developed the first pregnancy test—and scientists tested it in the 1960s and discovered it was accurate about 70% of the time.

Nasal congestion is a common side effect of pregnancy, called pregnancy rhinitis

In one recent study, almost 100% of pregnant women experienced this condition in their third trimester. The congestion is likely due to hormonal changes and neurophysiological factors, and it may also worsen at night due to circadian factors and gravitational redistribution when lying down. Treatments include saline nasal irrigation and the use of nasal strips to widen the nasal valve area.

Your brain appears to age in five distinct stages, with the pattern of connections inside your brain shifting, on average, around ages 9, 32, 66, and 83.

Read the 1963 study testing out an ancient Egyptian pregnancy urine test.

Advent's length varies from 21 to 28 days each year

Because Advent always begins on the fourth Sunday before Christmas, its duration varies. When Christmas falls on a Monday, Advent is unusually short, and when it falls on a Sunday, it can be nearly four full weeks long.

The halting of human bone growth helps manage heat produced from metabolism

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.

‘Hope for Healing’: The first-ever use of an in utero stem cell treatment

In 2023, researchers at UC Davis performed the first-ever fetal stem cell transplant, attempting to cure a case of spinal bifida prior to childbirth using stem cells derived from the placenta. The trial, and others including an effort to treat muscular dystrophy, highlight the potential power of stem cell therapy.

Menstruation, which occurs monthly, is the release of an unfertilized egg

This routine vaginal bleeding is part of a woman's hormonal cycle and occurs after an egg is released by an ovary during ovulation and isn't fertilized. Also known as periods, this monthly occurrence can cause a number of symptoms, including cramping, fatigue, and mood changes.

Cells can last in the human body from days to a person's entire life

Despite claims that a person's body replaces itself every seven to 10 years, the rate at which cells die and are replaced varies by cell type. Gastrointestinal cells that line the digestive tract and stomach last only a few days, while some heart muscle cells and most neurons are around from birth until death.

A timelapse of sprouting seeds

Germination is the process where an organism sprouts from a seed or spore. This timelapse captures the process in an inspiring, two-minute video so you can see the whole story of the seed quickly.

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.

What is Lent?

Lent is the 40-day period of fasting and penitence before Easter, modeled on Jesus's 40 days of fasting in the wilderness. Rooted in early Christian practice, it was formalized at the Council of Nicaea in 325 CE. Pope Gregory I established Ash Wednesday as its start in 601, creating the season's current structure of 46 days—40 fasting days plus six Sundays, which were traditionally treated as feast days.