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

Fetal development, week by week

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...

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.

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.

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

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

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

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 the US, 3.6 million babies were born in 2023

That year, about 40% of the babies were born to unmarried parents, and about 32% of the births were born via cesarean. On average, mothers were 27.5 years old when they had their first child.

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.

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.

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

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

Menopause is defined as 12 months without a period

Menopause is officially when a woman stops getting her period, meaning she has stopped ovulating and can no longer get pregnant. A woman has to go a full year without bleeding or spotting to know she has hit menopause.

Early texts claim Jesus was conceived on the same date of his death

Though celebrations of Jesus’ birth weren’t formalized for several hundred years after he lived, it was more common to commemorate his death. Some early Church writers connected the presumed date of his death—March 25—with his conception, implying a birth date of Dec. 25.

Lengthy incubation periods are why it's tough to link foodborne illness to specific foods.

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.

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.

Pregnancy makes someone more attractive to the mosquito species that transmits malaria.

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

Pregnant teachers were once banned from public school classrooms after their first trimester, with school boards believing that they were bad influences on students.

As of 2022, scientists estimate that 117 billion people have been born on Earth

Although no demographic data exist for more than 99% of the time humans have existed, this value can be estimated by tracking approximate population sizes over different periods and the birth rates within those periods. In 2022, about 6.8% of humans that have ever been born were alive.

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.

The most common birthdays

Nine of the top 10 most common birthdays fall in September, with Sept. 9 being the most common. However, August is the most common birth month, followed by July and December. On the flip side, the least common birthdays are around major holidays, with Christmas Day being the rarest—probably due to doctors scheduling fewer inductions and C-sections on those days. Find out more birthday surprises here.

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.

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.

Women are born with all the eggs they will ever produce in their lifetimes

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.

A credit card’s grace period is a minimum of 21 days

Under federal law in the United States, an individual’s credit card payment due date must fall on the same day of each month, at least 21 days after the cardholder’s statement closing date.

Premarin, the hormone medication derived from pregnant horses, is 'PREgnant MARes urINe'

Scientists in the late 1920s and early 1930s discovered estrogens could be extracted from human placenta, and then isolated it from the urine of pregnant women. But researchers discovered the urine of pregnant horses had similar substances and the horse product was much easier to collect in significant amounts—leading to Premarin.

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.

The latest data on US motherhood

Here’s a snapshot of what motherhood looks like in the U.S. today, drawn from government data and Pew Research Center surveys. The list includes seven headline statistics with visual graphs, including the average age of a mother's first childbirth...

The presidential election process continues for weeks after Election Day

Even after a winner is projected, the election is not complete. Between Election Day and Inauguration Day, electors formally cast votes and Congress meets in a joint session to certify the results, following a constitutionally defined timeline.

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.

Advent marks the beginning of the church calendar in Western Christianity

Each denomination’s calendar of holy days varies, but most mark the start of a new year at the beginning of Advent. Within Roman Catholicism, Advent is a distinct period from Christmas, alongside other seasons: Ordinary Time, Lent, Easter, Holy Week, and Pentecost.

Why Easter's date changes, making it a 'moveable feast'

Unlike Christmas, Easter has no fixed date: It falls on the first Sunday after the first full moon on or after March 21, a formula set by the Council of Nicaea in 325 CE to form a standard across Christianity. The result is a window of 35 possible dates, from March 22 to April 25.

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.

Babies don't develop a circadian rhythm right away

Newborns don't develop circadian rhythm until they are a few months old, which helps to explain their erratic sleep schedules. When they're around three months old they begin to release melatonin, which influences sleep, and around the same time they develop more regular sleeping routines.

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

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

Postpartum psychosis is an emergency requiring immediate psychiatric intervention, with symptoms including hallucinations, paranoia, and delusions that may put a baby at risk.

Chronological versus biological age

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).

Scientists can determine the age of rocks, fossils, and other ancient samples by measuring how many radioactive atoms in the sample have decayed, a process that occurs at precise, well-known rates.

China has used several different calendars because of changes in government structure

For many centuries, during which China was an imperial nation, the lunar/lunisolar calendar (aka the “rural calendar”) was the basis for organizing the year. Several attempts were made over the decades to switch to the Gregorian calendar—which begins Jan. 1—but it wasn’t until the People’s Republic of China was founded in 1949 that it stuck.

Since 1967, a second has been defined as how long it takes an electron to transition between two energy levels of a cesium-133 atom 9,192,631,770 times.

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 federal budget process begins nearly a year before funds are spent

Creating the federal budget starts months in advance. This resource explains each stage, showing how the president proposes priorities, Congress reshapes and approves funding, and federal agencies implement the final budget.

Calculate the salary of a mother's work

While traditional parenting roles are changing, mothers do a ton of work at home, from cleaning and cooking to managing household finances. This quick calculator estimates the salary a mother (or caregiver) should receive based on number of...

Postmenopause is a stage lasting roughly a third of a woman's life

Postmenopause is the final stage of menopause, occurring once a woman hasn't had any vaginal bleeding for a full year. She is then considered postmenopausal for the rest of her life. While some of the intense symptoms she might experience during postmenopause will decline, her risk for certain conditions and diseases will increase.

Perimenopause can last as long as 8 years

Perimenopause typically lasts about four years, beginning at around age 45; however, some women experience it as early as their 30s. Smoking and thyroid disease are factors that can trigger early perimenopause. Early menopause, which can increase health risks, occurs in about 5% of women. Researchers are still investigating why it occurs.

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 feet can expand—permanently—due to pregnancy.