Pregnancy Timeline

Your complete guide to pregnancy development from conception to delivery. This week by week timeline covers the key milestones, developmental changes, and what to expect during each stage of your pregnancy journey.

First Trimester (Weeks 1 to 12)

The first trimester is a period of rapid cellular development. Although you may not look pregnant yet, critical organ systems are forming during these early weeks.

Weeks 1 to 4: Conception and Implantation

Pregnancy is dated from the first day of your last menstrual period (LMP), even though conception typically occurs around week 2. By week 3, the fertilized egg travels down the fallopian tube and implants into the uterine lining. By week 4, the embryo is smaller than a grain of rice and the placenta begins to form.

Weeks 5 to 8: Early Organ Development

The heart begins beating around week 5. By week 6, the neural tube (which becomes the brain and spinal cord) closes. Tiny limb buds appear, and facial features begin to take shape. By week 8, all major organ systems have started forming. The embryo is now called a fetus and is about the size of a raspberry.

Weeks 9 to 12: Rapid Growth

Fingers and toes become distinct. The fetus begins to move, though you cannot feel it yet. By week 12, the fetus is about 2.5 inches long and weighs approximately half an ounce. The risk of miscarriage drops significantly after week 12. External genitalia begin to differentiate, and the first trimester screening window opens.

Second Trimester (Weeks 13 to 27)

Often considered the most comfortable trimester, this is a time of significant growth and increasing awareness of your pregnancy.

Weeks 13 to 16: Movement Begins

The fetus develops a fine layer of hair called lanugo. Facial muscles develop, allowing the fetus to make expressions. By week 14, the fetus can swallow, and the kidneys begin producing urine. Bone tissue starts replacing cartilage. The anatomy scan is typically scheduled between weeks 18 and 22.

Weeks 17 to 20: You May Feel Movement

Many women feel the first fluttering movements (quickening) between weeks 16 and 20, especially in their second pregnancy. By week 18, the fetus can hear sounds. By week 20, the halfway point, the fetus is about 10 inches long and weighs around 10 ounces. Vernix caseosa, a protective waxy coating, begins to cover the skin.

Weeks 21 to 27: Viability Milestone

The lungs begin developing surfactant, a substance critical for breathing after birth. The fetus develops regular sleep and wake cycles. By week 24, the fetus reaches the threshold of viability, meaning survival outside the womb is possible with intensive medical care, though outcomes vary. By week 27, the fetus weighs about 2 pounds and can open and close its eyes.

Third Trimester (Weeks 28 to 40)

The final trimester focuses on weight gain, lung maturation, and preparation for birth.

Weeks 28 to 32: Brain and Lung Development

The brain develops rapidly, forming billions of neurons. The fetus can now regulate its own body temperature. Fat layers accumulate under the skin, filling out the fetal body. The bones are fully developed but remain soft and flexible for delivery. By week 32, the fetus weighs about 3.5 to 4 pounds.

Weeks 33 to 36: Preparing for Birth

The fetus typically moves into a head down position in preparation for delivery. The lungs are nearly mature. Antibodies transfer from mother to fetus, providing immune protection for the first months of life. The fetus gains about half a pound per week during this period. By week 36, the fetus weighs approximately 5.5 to 6 pounds.

Weeks 37 to 40: Full Term

A pregnancy is considered full term at 39 weeks. The fetus continues to build fat reserves and practice breathing movements. The average newborn weighs between 6 and 9 pounds and measures 19 to 21 inches long. Labor typically begins between weeks 38 and 42. Only about 4 to 5 percent of babies arrive on their exact estimated due date.

Key Pregnancy Milestones at a Glance

  • Week 5: Heartbeat detectable on ultrasound
  • Week 8: All major organs are forming – the embryo becomes a fetus
  • Week 12: End of first trimester – miscarriage risk drops
  • Week 16 to 20: First fetal movements felt (quickening)
  • Week 18 to 22: Anatomy scan (mid pregnancy ultrasound)
  • Week 24: Viability milestone
  • Week 28: Third trimester begins
  • Week 37: Early term – lungs typically mature
  • Week 39 to 40: Full term – optimal time for delivery

Important Reminders

Every pregnancy is unique. The milestones described above represent typical development based on medical literature and guidelines from organizations, including the American College of Obstetricians and Gynecologists (ACOG). Your healthcare provider is the best source of information about your specific pregnancy.

This content is for educational purposes only and is not a substitute for professional medical advice. For more information, please review our Medical Disclaimer.

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