Fetal Development From Conception | Week-By-Week Milestones

From the moment an egg and sperm meet, a new life starts changing at a stunning pace. Cells divide, organs form, senses switch on, and day by day a tiny cluster of cells turns into a newborn ready to breathe, cry, and feed. Understanding how this process unfolds can make pregnancy feel less mysterious and far more manageable.

This guide walks through fetal growth from the very beginning, showing what happens at each stage, when milestones usually appear, and how everyday choices can help protect this delicate process. It does not replace medical care, but it can help you follow along with what your baby might be doing inside the womb each week.

Fetal Development From Conception Timeline Basics

Pregnancy is usually described as lasting around forty weeks from the first day of the last menstrual period. That date is called gestational age. The fertilization date comes about two weeks later for many pregnancies, so fetal age is often around two weeks younger than the gestational count. Medical sources such as the NICHD pregnancy overview describe this forty-one week window as the period when the baby grows and matures inside the uterus.

Specialists often divide this stretch into three trimesters. Weeks 1–12 make up the first trimester, weeks 13–27 the second, and weeks 28–40 the third. Each trimester has its own pattern. Early on, the focus is on building the basic structure. Later on, the focus shifts toward growth, fine tuning, and preparing for life outside the womb.

Gestational Age And Fetal Age

During the first eight weeks after fertilization, the baby is called an embryo. After that point, the same growing baby is called a fetus until birth. Guidance from the American College of Obstetricians and Gynecologists explains that the embryo label covers the earliest organ formation, while the fetal label covers the long period of growth that follows.

Most ultrasound reports and clinic visits refer to gestational age rather than fetal age. That is why a scan might say “ten weeks” even though fertilization likely happened around eight weeks ago. Both counting styles describe the same pregnancy; they just start the clock on different days.

Trimesters At A Glance

During the first trimester, the baby’s brain, spinal cord, heart, and major organs start forming. Facial features, limb buds, and tiny fingers and toes appear. By the end of this period the baby is fully “assembled” in miniature form.

During the second trimester, size increases, movements grow stronger, and senses such as hearing start coming online. Many parents first feel kicks in this period, often around twenty weeks. During the third trimester, the baby gains weight, stores fat, and fine-tunes organs such as the lungs in preparation for birth.

From Fertilization To Implantation

Every pregnancy starts when a sperm fertilizes an egg in the fallopian tube. The fertilized egg, called a zygote, carries half its genes from each parent. Within hours, that single cell begins to divide.

Over the next few days, the dividing ball of cells travels down the tube toward the uterus. Around day five, it becomes a blastocyst, a hollow ball with an inner cell mass that will form the baby and an outer layer that will form part of the placenta. Around day six to ten, the blastocyst attaches to the uterine lining and starts to sink in. This step, called implantation, allows the growing baby to tap into the parent’s blood supply for oxygen and nutrients.

Hormones And Early Symptoms

Once implantation begins, cells that will become the placenta release human chorionic gonadotropin, or hCG. This hormone supports the uterine lining and signals the body to stop shedding it each month. Rising hCG levels form the basis of most pregnancy tests.

During this time, many people notice tender breasts, fatigue, or light spotting. These early signs match the shift in hormone levels as the uterus prepares to host a growing embryo.

Embryonic Stage Weeks One To Eight

During the embryonic stage, the baby grows from a tiny disc of cells into a recognizable miniature body. Brain and spinal cord tissue begin to form in the first weeks. A simple heart tube starts to beat and pump blood. According to patient materials from Mayo Clinic on early fetal development, limb buds appear, and the foundations of eyes, ears, and internal organs develop during these first twelve weeks.

This period carries a higher risk of structural birth defects because organs are forming and shaping. A fact sheet from MotherToBaby, hosted by the U.S. National Library of Medicine, outlines how different exposures may have stronger effects at certain weeks, especially during organ formation in the first trimester. This is a major reason health agencies urge people to avoid alcohol, tobacco, and certain medications during early pregnancy, even before a test confirms it.

Formation Of Major Organs

By around six weeks of gestational age, a heartbeat can often be seen on ultrasound. Tiny buds that will become arms and legs grow longer, and ridges that will form fingers and toes appear. The face starts to show small eye spots and a forming mouth.

By around eight weeks, the basic structure of the brain, heart, lungs, liver, and kidneys is present. The baby is still only around an inch long from crown to rump, but the blueprint for the future body is in place.

Fetal Growth From Conception To Birth Stages

Once the embryonic stage ends, the fetal stage continues until delivery. At this point the focus shifts from building new organs to growing, maturing, and refining what already exists. An overview from the ACOG fetal growth FAQ notes that from nine weeks onward, the baby’s organs enlarge and take on more complex functions while features such as fingernails, eyebrows, and hair appear.

Each trimester has a different feel. The first trimester can bring nausea and fatigue, the second often brings more energy and rounder curves, and the last can come with a heavier belly and stronger kicks. The table below sums up these stages in one view.

Time Period	Baby’s Development	What You Might Notice Or Do
Preconception And Weeks 1–2	Eggs mature in the ovaries; sperm and egg have not yet met.	Plan ahead with folic acid and health checks if pregnancy is possible.
Weeks 3–4	Fertilization, early cell division, and implantation in the uterus.	Possible light spotting, mild cramps, or no symptoms at all.
Weeks 5–6	Heart tube begins beating; early brain and spinal cord structures form.	Positive pregnancy test, breast tenderness, nausea, or strong fatigue.
Weeks 7–8	Limb buds lengthen; facial features and inner organs take shape.	First prenatal visit often scheduled; early ultrasound may be done.
Weeks 9–12	Embryo is now called a fetus; fingers, toes, and joints become clearer.	Nausea may ease near the end of this window; clothing may feel tighter.
Weeks 13–20	Skeleton hardens; baby starts swallowing, kicking, and hearing sounds.	More energy, first flutters of movement, anatomy scan around twenty weeks.
Weeks 21–28	Rapid weight gain; lungs and brain structures grow more complex.	Stronger kicks, possible heartburn, glucose screening for gestational diabetes.
Weeks 29–36	Body fat builds; baby practices breathing motions and settles into position.	Backache, sleep changes, more frequent checkups, planning for birth.
Weeks 37–40	Full-term baby with mature lungs, ready for labor and delivery.	Regular visits, signs of labor, and final preparations for meeting the baby.

First Trimester Highlights

By the end of week twelve, many sources describe the baby as “fully formed,” with all major organs present in miniature. The head is large compared with the rest of the body because brain growth is so active at this time. Fingers and toes separate, and tiny fingernails begin to grow.

On the parent’s side, early symptoms may fade as hormone levels adjust. Around this time some people choose to share news of the pregnancy more widely, often after first-trimester screening tests come back.

Second Trimester Highlights

During weeks thirteen through twenty-seven, the baby’s body stretches out and gains strength. Many parents feel the first gentle kicks between sixteen and twenty weeks. The baby can move joints, flex fingers, and respond to loud sounds. According to week-by-week guides from groups such as March of Dimes, the skin is thin and covered with a fine hair called lanugo during this phase, and the baby starts to practice swallowing amniotic fluid.

On ultrasound, the face looks more familiar, with a defined nose, lips, and eyelids. Many clinics schedule a detailed anatomy scan around the mid-point of pregnancy to check organ structure and growth.

Third Trimester Highlights

From week twenty-eight until birth, weight gain speeds up. The baby lays down fat under the skin, which helps with warmth after delivery. The lungs produce more surfactant, a substance that helps tiny air sacs stay open after the first breath. The brain forms more folds and connections.

The baby usually turns head-down during this period, although exact timing varies. Movements can feel sharper as space grows tighter. Many parents notice shortness of breath, swelling in the feet or hands, and more frequent bathroom trips as the uterus presses on nearby organs.

Critical Windows And Prenatal Care

Some weeks carry higher sensitivity to certain exposures than others. The MotherToBaby critical periods fact sheet describes how organ systems each have their own window when they form and can be altered by harmful substances. For instance, the heart and neural tube have early windows in the first trimester, while the brain and growth patterns continue to adjust throughout pregnancy.

Regular prenatal care gives health professionals a chance to track these phases. Blood tests, ultrasound scans, and screening tools help confirm that growth stays on track and that the parent’s health supports the pregnancy. Guidance from the CDC on planning and pregnancy stresses the value of early visits, folic acid, and management of chronic conditions before and during pregnancy.

Screenings Across The Months

In early pregnancy, common tests include blood type, iron levels, and screening for infections. Some families also choose genetic screening or diagnostic tests that look for specific conditions. During the second trimester, an anatomy ultrasound checks organ structure and the placement of the placenta.

Later in pregnancy, tests often include glucose screening for gestational diabetes and checks for high blood pressure or preeclampsia. These conditions relate to the environment in which the baby grows, so catching them early helps protect both parent and baby.

How Lifestyle Choices Shape Fetal Growth

From conception onward, the baby depends on the parent’s body for oxygen, nutrients, and waste removal. That connection means everyday habits can shape growth for better or worse. Health agencies across the world encourage people who are pregnant, or who might become pregnant, to aim for steady nutrition, gentle movement, and avoidance of known harmful substances.

Resources such as CDC guidance for pregnancy and March of Dimes pregnancy resources outline these steps in more detail, from vaccine choices to food safety and mental health care. They also stress that every pregnancy is different, so individual care plans may vary.

Nutrition, Folic Acid, And Supplements

Before and during pregnancy, folic acid plays a central role in reducing the risk of neural tube defects. The CDC recommends at least four hundred micrograms per day starting at least one month before conception when possible. Many prenatal vitamins supply this amount along with iron, iodine, and other nutrients that support fetal development.

A balanced eating pattern with fruits, vegetables, whole grains, lean proteins, and healthy fats helps supply the building blocks for new tissues. Regular meals and snacks can also help manage nausea and blood sugar swings. People with special diets or health conditions may need individual advice on supplements or adjustments.

Substances And Exposures To Avoid

Alcohol, tobacco, and illicit drugs can harm a developing baby at many stages. Some prescription or over-the-counter medicines also carry risks during pregnancy. Because every case is different, it is wise to review current medicines with a health care professional when planning a pregnancy or soon after a positive test.

Certain infections, high fevers, and environmental exposures such as lead or some solvents can also influence fetal growth. Public health sites and local clinics often provide lists of known risks and steps to reduce exposure at home and work.

Sample Week Landmarks In Pregnancy

No two pregnancies match week by week, yet some patterns appear often enough to give parents a helpful timeline. Medical references such as Cleveland Clinic and other obstetric guides describe common ranges for length, weight, and abilities at different points in gestation. The table below offers a simplified picture that many families find handy while reading scan reports or pregnancy apps.

Week	Average Size	Notable Change
4	Poppy seed, only a few millimeters long	Implantation completes; basic neural tube starts forming.
6	Lentil, around one-quarter inch long	Heartbeat present; tiny limb buds visible on ultrasound.
8	Kidney bean, around three-quarters inch long	Facial features and fingers begin to appear; tail recedes.
12	Lime, around two and a half inches long	All major organs formed; baby can move arms and legs.
20	Banana, around ten inches from head to heel	Many parents feel strong kicks; anatomy scan often done.
28	Eggplant, around fourteen inches long	Eyes can open and close; baby can respond to light and sound.
32	Squash, around sixteen and a half inches long	More body fat; movements may feel rolling and strong.
38	Small watermelon, around eighteen to twenty inches long	Full-term or near full-term baby ready for labor and birth.

Watching Fetal Development During Prenatal Visits

Routine visits allow parents to follow fetal development from conception through every trimester. Early in pregnancy, these visits confirm the location of the pregnancy, estimate due dates, and pick up any conditions that might need extra monitoring. Later on, they track growth, fluid levels, and the baby’s position.

Health professionals use a mix of tools: fundal height measurements, Doppler devices to hear the heartbeat, and ultrasound scans at key stages. In some cases, extra tests help keep a closer eye on growth or blood flow in the placenta. These checkups also provide space to ask questions and plan for labor, feeding, and life with a newborn.

When To Call A Health Care Professional

Pregnancy comes with many new sensations, so it can be hard to know what falls inside the normal range. Sudden severe pain, heavy bleeding, fluid leaking from the vagina before term, a marked drop in fetal movement after twenty-eight weeks, or signs of high blood pressure such as severe headache or vision changes all deserve prompt attention.

Local emergency numbers and maternity triage lines can guide next steps when concerning symptoms appear. Every region has its own systems, so clinics often provide a list of phone numbers at the first visit. Keeping that list handy gives parents a clear plan for how to act if something feels wrong during any stage of fetal development from conception through birth.