Fetal Development Stages | Week-By-Week Growth Markers

People talk about pregnancy in weeks for a reason: the changes stack fast, and each week builds on the last. This guide walks through fetal development stages from conception to birth in plain language. You’ll see what’s forming, when major body systems start working, and why your clinician may date a pregnancy in a way that feels off by two weeks.

How pregnancy weeks are counted

Most clinics use gestational age, counted from the first day of the last menstrual period. Conception often happens about two weeks later, depending on cycle timing. That’s why “6 weeks pregnant” often means the embryo has been developing for about four weeks since fertilization.

Dating can shift after an early ultrasound. That’s normal. It reflects how pregnancy care is scheduled, not a judgment about when you conceived.

Fetal Development Stages by trimester and week markers

Development is often described in three phases. The phase names are less common in day-to-day chat, yet they match what’s happening in the body.

Germinal stage

This runs from fertilization through implantation. The fertilized egg divides, travels down the fallopian tube, then attaches to the uterine lining. During this window, a person may not feel any changes at all, or may notice light spotting around implantation.

Embryonic stage

This stage spans most of weeks 5–10 by gestational age. The basic body plan takes shape: brain and spinal cord begin forming, the heart starts beating, and early limb buds appear. Many pregnancy symptoms start here because hormone levels rise quickly.

Fetal stage

From about week 11 to birth, growth and organ maturation take center stage. Structures that formed earlier get refined: bones harden, lungs develop, and the brain builds more complex connections. Movement becomes easier to feel as the fetus gains strength and space changes inside the uterus.

Week-by-week overview of fetal development

This timeline uses the common clinical week count (gestational age). Milestones can vary by days to a couple of weeks and still fit a healthy range. If you want a clinician-written snapshot by month, the American College of Obstetricians and Gynecologists shares a month-to-month overview in ACOG’s “How Your Fetus Grows During Pregnancy”.

Weeks 1–4

Fertilization happens, the early cell ball implants, and the placenta starts forming. By the end of week 4, the embryo is tiny, yet the groundwork for the nervous system is starting.

Weeks 5–6

The neural tube is closing, the heart begins to beat, and facial features start to outline. Early ultrasound may show a gestational sac and yolk sac; a heartbeat may be seen toward the end of this window.

Weeks 7–8

Arms and legs lengthen, fingers and toes begin separating, and the brain grows rapidly. Tiny cartilage forms that will later become bones.

Weeks 9–10

Major organs are present in basic form. External ears and eyelids develop. The embryo starts to move, though movement is not yet felt by the pregnant person.

Weeks 11–13

The fetus enters the fetal stage. Bones start hardening, the intestines move into the abdomen, and facial features look more defined. Many people notice nausea easing as the second trimester approaches.

Weeks 14–16

Neck strength improves, the fetus can make sucking motions, and fine hair begins to appear later in this window. Some people start to feel faint flutters, especially in later pregnancies.

Weeks 17–20

Hearing structures develop further, sleep-wake patterns begin to show, and movement becomes stronger. Many clinics schedule an anatomy ultrasound around this time to check growth and structures.

Weeks 21–24

Lung development continues, with air sacs forming and surfactant production beginning later in this span. Skin is thin and more translucent. With each week, medical teams have more options to care for babies born early.

Weeks 25–28

Brain growth accelerates, eyes can open, and the fetus gains body fat. Third trimester begins near the end of this range.

Weeks 29–32

The fetus practices breathing motions, and the immune system receives antibodies from the pregnant person. Kicks and stretches can feel stronger as space gets tighter.

Weeks 33–36

Weight gain speeds up. Many babies move into a head-down position during these weeks. Nails and hair continue to grow.

Weeks 37–40

Organs are ready for life outside the uterus, with lungs among the last to finish maturing. The baby keeps adding fat and polishing coordination, feeding skills, and temperature control.

Milestones by time window

Sometimes you need a fast way to map “what forms when” without reading 40 separate week entries. The table below compresses the broad arc while still staying specific.

Time window	What’s forming or maturing	What many people notice
Weeks 1–4	Implantation, early placenta, early nervous system groundwork	Often no symptoms or mild spotting
Weeks 5–6	Neural tube closure, early heart activity, facial outline	Fatigue, nausea, breast tenderness
Weeks 7–8	Limb growth, early cartilage, rapid brain growth	Smell sensitivity, food aversions
Weeks 9–10	Organs present in basic form, early movement	Morning sickness often peaks
Weeks 11–13	Bone hardening begins, features refine	Some symptom relief for many
Weeks 14–20	Stronger movement, hearing development, anatomy scan window	Energy may rise; flutters may start
Weeks 21–28	Lung air sacs develop, surfactant begins, fat gain starts	More distinct kicks; belly growth
Weeks 29–36	Practice breathing, antibody transfer, rapid weight gain	Heartburn, back pressure, sleep shifts
Weeks 37–40	Final maturation, fat stores, feeding coordination	Pelvic pressure; nesting feelings

Why the first trimester gets special attention

Many organs begin forming early. That’s why clinicians talk so much about early prenatal care and nutrient intake. One standout nutrient is folic acid, linked with lower risk of neural tube defects when taken before conception and during early pregnancy. The Centers for Disease Control and Prevention explains doses and timing on CDC’s “Folic Acid: Facts for Clinicians”.

If you’re planning pregnancy, this is also the window when many people review medications, alcohol intake, and workplace exposures with a clinician. If you’re already pregnant, bring a current list of medicines and supplements to your first visit so your care team can check safety.

Embryo to fetus: what changes at week 11

The word change reflects a shift in focus. By week 11, most structures are present. After that, growth, movement, and organ function become the big themes. This is also when many screening and dating steps happen, since ultrasound images are clearer.

How growth is tracked in prenatal care

Clinics track growth with a mix of dates, measurements, and lab results. You’ll see a few repeated terms:

• Crown-rump length. Used in early ultrasound to estimate gestational age.
• Fundal height. A tape measure check from pubic bone to top of the uterus, often starting mid-pregnancy.
• Estimated fetal weight. A calculation from ultrasound measurements that gives a range, not a single exact number.

These tools flag patterns that might need a closer look. They do not predict the exact birth weight or birth date.

Normal variation you can expect

Two pregnancies at the same week can look different on scan. A fetus may measure a few days ahead or behind. Clinicians care more about the growth trend across visits than one data point.

Prenatal tests by stage

Testing schedules differ by country and by medical history. Still, many systems follow a similar rhythm. MedlinePlus has a week-by-week overview that matches this general timeline in “Fetal development”.

When it’s often offered	What it can check	What you’ll usually do
Early first trimester	Dating, number of embryos, early anatomy	Ultrasound
First trimester	Chromosome screening options (varies)	Blood test, sometimes ultrasound markers
Mid second trimester	Structural anatomy, placenta position	Anatomy ultrasound
Second trimester	Diabetes screening (timing varies)	Glucose drink test
Third trimester	Anemia, infections (timing varies)	Blood tests
Late third trimester	Group B strep screening (common in many places)	Swab test
Any trimester	Growth checks if symptoms or risk factors arise	Targeted ultrasound, monitoring

Daily choices that shape healthy development

This section sticks to broad, evidence-backed habits that most prenatal care teams talk about. For detailed visit schedules and what care can include, the World Health Organization lays out antenatal care recommendations in “WHO recommendations on antenatal care for a positive pregnancy experience”.

Nutrition basics

Pregnancy needs steady energy and nutrients, yet “eating for two” is often misunderstood. Many people do better with small meals, protein at breakfast, and snacks that include fiber and fluids. Prenatal vitamins are commonly used to fill gaps, with folic acid early on as one widely recommended piece.

Movement and rest

Regular movement can ease back pain, constipation, and sleep issues for many people. Choose options that feel steady and safe for your body: walking, swimming, prenatal yoga, or light strength work. If you have bleeding, severe pain, or dizziness, pause and contact your care team.

Substances and exposures

Alcohol, nicotine, and recreational drugs can harm fetal growth. Many medications are safe, yet some are not. Bring each prescription, over-the-counter med, and supplement to a prenatal visit for a personal review. Also check labels for vitamin A dosing, since high amounts can be risky.

Red flags that need same-day medical advice

Call your maternity unit, urgent care line, or local emergency number if you have any of these symptoms:

• Heavy bleeding, clots, or strong cramping
• Severe headache, vision changes, chest pain, or shortness of breath
• Fluid leaking from the vagina
• Fever with chills
• Marked drop in fetal movement after you’ve been feeling regular movement

These signs can have benign causes, yet they also can signal conditions that need rapid evaluation.

Trimester milestone checklist

If you want one page of notes to save on your phone, use this checklist and add your own dates from your clinician.

First trimester

• Confirm dating and due date estimate
• Start prenatal vitamins that include folic acid
• Review meds and supplements with your care team
• Plan screening choices that fit your values and history

Second trimester

• Schedule anatomy ultrasound if it’s part of your care plan
• Track movement patterns once you begin feeling them
• Talk about sleep, back pain, and activity options that feel good

Third trimester

• Ask about signs of labor and when to call
• Confirm newborn care plan: feeding, car seat, pediatric follow-up
• Review late-pregnancy testing and vaccination timing if offered

How this guide was built

The week markers and phase descriptions come from clinician-facing and public-health sources. The language is kept broad because pregnancy care is individualized. Use this page to understand the arc of fetal development stages, then match the details to your own care plan with your clinician.