Fetal Development Week by Week: Pregnancy Milestones Explained

Fetal development is a continuous process that begins with fertilization and continues throughout pregnancy. During approximately 40 weeks of gestation, a fertilized egg develops into an embryo and then a fetus with increasingly mature organs, body systems, movements, and senses.

This week-by-week guide explains the major pregnancy milestones from conception to birth. Developmental timing can vary, so the weeks below should be understood as general reference points rather than exact deadlines.

How Are Pregnancy Weeks Calculated?

Gestational age describes how far a pregnancy has progressed. It is usually measured from the first day of the pregnant person’s last menstrual period.

This dating method begins approximately two weeks before fertilization in a typical menstrual cycle. Therefore:

• Week 1 begins on the first day of the last menstrual period.
• Week 2 generally includes the period leading up to ovulation.
• Week 3 is often when fertilization occurs.
• Week 4 is commonly when implantation is completed and a pregnancy test may become positive.

Menstrual cycles vary, so conception does not always occur on the same cycle day. Healthcare professionals may use an early ultrasound to establish or confirm gestational age.

What Is the Difference Between an Embryo and a Fetus?

The developing baby is described using different terms according to the stage of development:

• Zygote: the single cell created when a sperm fertilizes an egg.
• Blastocyst: a developing cluster of cells that travels toward the uterus and implants in the uterine lining.
• Embryo: the stage during which the basic structure of the body and major organ systems begin to form.
• Fetus: the later stage characterized primarily by growth, refinement, and maturation of the organs and body systems.

Medical definitions may describe the fetal stage according to time since fertilization or gestational dating. In everyday pregnancy guidance, the transition is commonly discussed around the end of the embryonic period during the first trimester.

Weeks 1–4: Fertilization and Implantation

Weeks 1 and 2

During the first two weeks of gestational dating, pregnancy has not usually begun biologically. The body is preparing for ovulation by developing an ovarian follicle and building the uterine lining.

Week 3

Fertilization usually occurs in a fallopian tube when a sperm joins an egg. The resulting zygote begins dividing repeatedly as it travels toward the uterus.

Over several days, the dividing cells form a blastocyst. The blastocyst contains cells that will contribute to the embryo and cells that will help form supporting structures such as the placenta.

Week 4

The blastocyst attaches to and becomes embedded in the uterine lining in a process called implantation. Hormonal signals help maintain the uterine lining and support the developing pregnancy.

The early amniotic sac, yolk sac, and placental structures begin developing. A home pregnancy test may become positive as levels of human chorionic gonadotropin, commonly called hCG, rise.

Weeks 5–8: Early Embryonic Development

Weeks 5 through 8 are a period of rapid and complex development. The foundations of the brain, spinal cord, heart, face, limbs, lungs, digestive system, and other organs begin to form.

Week 5

• The neural tube begins developing into the brain and spinal cord.
• Early cardiac tissue develops.
• The placenta and umbilical connection continue forming.
• The embryo has a curved shape and is extremely small.

Week 6

• Early structures of the brain continue to develop.
• Small limb buds appear where the arms and legs will form.
• The inner ear and facial structures begin taking shape.
• Cardiac activity may sometimes be visible on ultrasound, depending on dating and equipment.

Week 7

• The brain grows rapidly.
• The face becomes more defined.
• Developing hands and feet become more noticeable.
• The liver, digestive tract, and other internal structures continue forming.

Week 8

• Arms and legs become longer.
• Developing fingers and toes begin separating.
• The eyes, ears, nose, and upper lip become more recognizable.
• The major organ systems have started forming, although they remain immature.

Weeks 9–12: Transition to the Fetal Stage

During this period, development shifts from the initial formation of body structures toward continued growth and increasing refinement.

Week 9

• The head remains relatively large compared with the body.
• The hands, feet, and early joints continue developing.
• The face becomes increasingly recognizable.
• Major internal organs continue developing.

Week 10

• The developing baby is entering the fetal stage.
• The limbs can bend at developing joints.
• Fingers and toes are more clearly separated.
• Facial features and external ears become more distinct.

Week 11

• The body begins to lengthen.
• The genital structures continue developing.
• Small spontaneous movements occur, although they are not yet felt.
• The bones and muscles continue maturing.

Week 12

• The internal organs and muscles continue growing.
• The skeleton begins gradually hardening from softer tissue into bone.
• The kidneys begin contributing to urine production.
• Cardiac activity can normally be evaluated with appropriate prenatal equipment.

Weeks 13–16: Growth, Movement, and Facial Development

The second trimester begins around this stage. The fetus grows rapidly, the body becomes more proportionate, and movement becomes increasingly coordinated.

Week 13

• The fetus moves inside the uterus, although these movements may not yet be felt.
• The arms and legs become more developed.
• The external reproductive structures continue differentiating.
• The face continues gaining definition.

Week 14

• Facial muscles begin making small movements.
• The neck becomes more defined.
• The arms and legs move more freely.
• The liver and spleen continue performing developing functions.

Week 15

• A fine layer of hair called lanugo begins developing on the body.
• Eyebrows and eyelashes begin forming.
• The eyes may become sensitive to light despite remaining closed.
• The auditory system continues developing.

Week 16

• The head becomes more upright.
• Facial features are increasingly recognizable.
• The limbs and joints are well formed.
• Some pregnant people may begin noticing early fluttering movements, particularly in later pregnancies.

Weeks 17–20: Movement, Hearing, and the Anatomy Scan

Week 17

• The fetus becomes more active.
• The eyes can move while the eyelids remain closed.
• Developing hearing may allow responses to sound.
• Fingerprints and fingernails continue forming.

Week 18

• The ears are approaching their final position.
• Movement becomes more coordinated.
• The digestive system continues practicing swallowing.
• Some pregnant people begin feeling fetal movement, known as quickening.

Week 19

• A protective substance called vernix caseosa begins covering the skin.
• The sensory areas of the brain continue developing.
• The arms and legs become increasingly proportionate.
• Movements may become more noticeable.

Week 20

• The pregnancy is approximately halfway through a 40-week timeline.
• Fetal movement may feel like fluttering, tapping, or gentle kicks.
• The scalp, nails, and hair follicles continue developing.
• A detailed ultrasound is often performed around the middle of pregnancy.

Weeks 21–24: Developing Senses and Lungs

Week 21

• Fetal movements become stronger and more organized.
• The fetus swallows amniotic fluid.
• The digestive system continues practicing future functions.
• Bone marrow gradually contributes more to blood-cell production.

Week 22

• The eyebrows and eyelids are well defined.
• The senses continue maturing.
• The lungs develop increasingly complex airways.
• The skin remains thin and wrinkled because body fat is still limited.

Week 23

• Hearing continues to improve.
• The fetus may respond to loud sounds or movement.
• The lungs begin producing small amounts of substances involved in keeping future air sacs open.
• Periods of activity and rest become more recognizable.

Week 24

• The lungs remain immature but continue developing rapidly.
• The brain and nervous system become increasingly complex.
• The fetus continues gaining length and weight.
• Premature birth at this stage requires highly specialized neonatal care and carries substantial medical risks.

Discussions about viability should always be individualized. Outcomes depend on gestational age, fetal growth, medical circumstances, available neonatal care, treatment decisions, and other factors.

Weeks 25–28: Brain, Eyes, and Nervous-System Development

Week 25

• The brain continues forming new connections.
• The fetus may respond to familiar sounds.
• Reflexes and coordinated movements continue improving.
• Body fat slowly begins accumulating.

Week 26

• The eyelids may begin opening.
• The auditory system becomes more responsive.
• The lungs continue producing surfactant-related substances.
• Breathing-like movements occur as preparation for life after birth.

Week 27

• The nervous system gains greater control over body functions.
• Sleep and wake cycles become more established.
• The fetus may respond to light, sound, and movement.
• Growth and fat storage continue.

Week 28

• The third trimester begins around this stage.
• The eyes can open and close.
• The brain develops rapidly and becomes more complex.
• The fetus continues practicing breathing movements.

Weeks 29–32: Rapid Growth and Organ Maturation

Week 29

• The muscles and lungs continue maturing.
• The head grows to accommodate the developing brain.
• Movements may feel stronger as the fetus gains muscle.
• Calcium and other nutrients support continued bone development.

Week 30

• The brain develops grooves and folds that increase its surface area.
• Body fat increases.
• The bone marrow plays a larger role in producing blood cells.
• The eyes may respond to changes in light.

Week 31

• The lungs and nervous system continue maturing.
• The fetus practices sucking and swallowing.
• Movements remain frequent, although the type of movement may change as space becomes limited.
• Fat storage helps the skin appear less transparent.

Week 32

• Most major body structures are formed.
• Development now focuses heavily on growth and maturation.
• The lungs continue preparing for breathing after birth.
• Some fetuses begin settling into a head-down position.

Weeks 33–36: Preparing for Birth

Week 33

• The bones continue hardening, although the skull remains flexible.
• The immune system receives antibodies from the pregnant parent.
• The brain and lungs continue maturing.
• Body fat increases rapidly.

Week 34

• The lungs are more mature but are still developing.
• The fingernails may reach the fingertips.
• The fetus continues gaining weight.
• The central nervous system increasingly coordinates breathing and temperature-related functions.

Week 35

• The kidneys are well developed.
• The liver continues maturing.
• The fetus occupies more space within the uterus.
• Movement may feel different but should remain present.

Week 36

• The fetus continues gaining fat and muscle.
• Many fetuses are positioned head-down, although position can vary.
• The lungs, brain, and digestive system continue maturing.
• Prenatal visits often become more frequent near the end of pregnancy.

Weeks 37–40: Term Pregnancy and Final Maturation

Although 37 weeks was historically described simply as “term,” modern terminology separates the final weeks because important brain, lung, liver, and nervous-system development continues.

Week 37

• The fetus continues practicing breathing, sucking, and swallowing.
• The brain and lungs continue developing.
• Fat stores support temperature regulation after birth.
• The fetus may descend lower into the pelvis.

Week 38

• The organs continue final maturation.
• The grasp reflex is strong.
• Most lanugo has disappeared, although some may remain.
• The placenta continues transferring oxygen and nutrients.

Week 39

• The pregnancy is considered full term.
• The brain, lungs, and liver continue developing.
• The chest becomes more prominent.
• The fetus continues gaining some weight.

Week 40

• The estimated due date is reached.
• The fetus is generally ready for life outside the uterus.
• The skull bones remain flexible to help during vaginal birth.
• Delivery may occur before or after the estimated date.

An estimated due date is not a guaranteed delivery date. Healthcare professionals consider gestational age, maternal health, fetal well-being, cervical changes, labor symptoms, and other clinical factors when planning care.

How Is Fetal Development Monitored?

Prenatal care helps healthcare professionals assess the progress of pregnancy and identify concerns that may require further evaluation.

Monitoring may include:

• Reviewing the date of the last menstrual period
• First-trimester ultrasound dating
• Measuring fetal cardiac activity
• Checking uterine growth or fundal height
• Ultrasound assessment of fetal anatomy and growth
• Monitoring fetal movement patterns
• Checking placental location and amniotic fluid
• Additional testing for pregnancies with specific medical risks

Not every pregnancy requires the same tests or the same number of ultrasound examinations. Care is adapted to medical history, pregnancy progress, local healthcare guidelines, and the needs of the pregnant person and fetus.

Fetal Development by Trimester

Frequently Asked Questions

Why are the first two weeks counted before conception?

Pregnancy is traditionally dated from the first day of the last menstrual period because this date is often easier to identify than the exact date of ovulation or fertilization. In a typical cycle, fertilization occurs approximately two weeks later.

When does the embryo become a fetus?

The terminology changes near the end of the embryonic stage, after the basic body structures and organ systems have begun forming. Exact descriptions may differ depending on whether development is measured from fertilization or by gestational age.

When can fetal cardiac activity be detected?

Cardiac activity may sometimes be visible on an early ultrasound during the first trimester. The timing depends on the true gestational age, the type of ultrasound, equipment quality, fetal position, and other factors. An early scan that does not show expected findings may need to be repeated.

When can a pregnant person feel the baby move?

First movements are often noticed during the second trimester, commonly around 18 to 20 weeks, although they may be felt earlier or later. People who have previously been pregnant may recognize movement sooner.

Is fetal development exactly the same in every pregnancy?

No. Development follows a general sequence, but exact timing, fetal size, movement patterns, and ultrasound measurements naturally vary. Healthcare professionals interpret findings using validated reference ranges and the overall clinical context.

Is 37 weeks considered full term?

Thirty-seven weeks through 38 weeks and 6 days is classified as early term. Full term begins at 39 weeks and continues through 40 weeks and 6 days.

What should I do if fetal movement decreases?

Contact your maternity unit, midwife, obstetrician, or other prenatal healthcare professional promptly if the fetus is moving less than usual or the normal movement pattern changes significantly. Follow the instructions provided by your local maternity service.

Final Thoughts

Fetal development is a remarkable process that progresses from a single fertilized cell to a newborn capable of breathing, feeding, moving, and adapting to life outside the uterus.

The first trimester focuses heavily on the formation of essential body structures. The second trimester brings rapid growth, developing senses, and increasingly noticeable movement. During the third trimester, the brain, lungs, nervous system, and other organs continue maturing while the fetus gains weight and prepares for birth.

Week-by-week timelines are useful educational tools, but they cannot replace individualized prenatal assessment. Regular prenatal care remains the best way to monitor the health and progress of a pregnancy.

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