How pregnancy dating begins
Pregnancy is counted from the first day of your last menstrual period, so gestational week 1 begins before conception.
PREGNANCY, ONE WEEK AT A TIME
Your week-by-week pregnancy guide
A calm, clear look at how pregnancy is dated and how development unfolds from week 1 through week 40.
Why week 1 starts before conception
A pregnancy is usually dated from the first day of the last menstrual period. That adds roughly two weeks before fertilization to the count and creates the familiar 40-week timeline. Your care team may use an early ultrasound to confirm or refine dating when needed.
Weeks 1–13
First trimester
Pregnancy dating begins before conception, followed by fertilization and the early formation of major organs and body systems.
The body prepares
Ovulation often happens near the end of this week in a typical cycle, although cycle timing varies.
Fertilization and cell division
If fertilization occurs, the resulting cell begins dividing while traveling toward the uterus.
Implantation begins
The developing cluster of cells reaches the uterus and begins attaching to its lining.
Foundations take shape
The brain, spinal cord and heart begin developing during this rapid period of early formation.
Early structures emerge
Arm and leg buds appear, and the developing brain and heart continue changing quickly.
Growth accelerates
The brain forms distinct areas while early facial, eye and ear structures continue to develop.
Hands, feet and lungs begin
The limbs lengthen, hands and feet begin forming, and the lungs start to develop.
Features become clearer
Elbows develop, toes become visible and all essential organs have started forming.
The face becomes more distinct
Eyelids and outer ears take shape while internal systems continue their early development.
Small details develop
Fingers and toes become more defined as the body continues a period of very rapid growth.
The first trimester nears its close
Major structures are present and will keep growing and maturing throughout the months ahead.
Continued growth and maturation
Bones, muscles and organs continue developing as the first trimester comes to an end.
Weeks 14–27
Second trimester
This is a period of rapid growth and development. Movement often becomes noticeable, and the standard anatomy ultrasound is commonly performed.
The second trimester begins
Growth becomes the central story of this trimester, with bones and muscles continuing to strengthen.
Bones and muscles develop
The skeleton continues hardening and coordinated movement becomes increasingly possible.
Movement becomes more coordinated
The arms and legs can move more purposefully, even if you cannot feel that movement yet.
Fat stores begin forming
Early body fat starts developing and the skeleton continues changing from softer tissue to bone.
A detailed look is approaching
The ears and limbs are more developed; a standard anatomy ultrasound is often scheduled between 18 and 22 weeks.
Protective layers develop
A protective coating called vernix begins forming on the skin as sensory systems continue developing.
The halfway point
Movement may be easier to notice around this stage, though the timing of first movement varies widely.
Practice and repetition
Swallowing and movement help the developing body rehearse functions it will use after birth.
Facial details sharpen
Eyebrows and eyelids are more defined while the lungs continue their long process of development.
Patterns begin to emerge
Periods of activity and rest become more noticeable as movement and coordination increase.
The lungs keep branching
The lower airways continue developing, while the skin remains thin and delicate.
Responses grow stronger
Movement may feel more distinct as the nervous system and muscles continue working together.
Eyes and lungs mature
The eyelids can begin opening, and the lungs continue producing substances needed for breathing after birth.
Brain growth continues
The brain and nervous system mature rapidly as the second trimester reaches its final week.
Weeks 28–40
Third trimester
Weight gain increases and organs mature as the body prepares for birth and life outside the uterus.
The third trimester begins
This trimester centers on weight gain and organ maturation in preparation for life after birth.
Strength and growth
Muscles and lungs continue maturing, and movements may feel stronger or more recognizable.
Eyes open and close
The eyes can open and close while the body continues adding fat and building blood cells.
Rapid weight gain
Growth speeds up and additional fat begins smoothing the skin and supporting temperature regulation.
Breathing movements continue
Rhythmic breathing movements occur, although the lungs are still developing.
Mineral stores build
The body stores iron, calcium and phosphorus while bones remain developed but relatively soft.
More growing, more maturing
The brain, lungs and nervous system continue developing as body fat steadily increases.
Sleep patterns are established
Periods of sleep and wakefulness are more defined while weight gain continues.
Preparing for birth
Many babies settle into a head-down position by this stage, although position can still vary.
Approaching term
Growth continues and the lungs, brain and liver keep maturing during these final weeks.
Finishing touches
Most fine body hair has disappeared, while fingernails and hair may continue growing.
The full-term window
At 39 weeks, pregnancy is considered full term; the brain and lungs still benefit from ongoing development.
Your estimated due week
A due date is an estimate, not an appointment—only about 1 in 20 people give birth on that exact date.
Sources and editorial note
Reviewed against established clinical and public-health references. Milestones describe typical development and may not match every ultrasound or pregnancy exactly.
- American College of Obstetricians and Gynecologists: How Your Fetus Grows During Pregnancy
- MedlinePlus: Fetal Development
- American College of Obstetricians and Gynecologists: Ultrasound Exams
- American College of Obstetricians and Gynecologists: Definition of Term Pregnancy
- American College of Obstetricians and Gynecologists: When Pregnancy Goes Past Your Due Date