How to Track Pregnancy Week by Week

Pregnancy unfolds across 40 weeks of remarkable transformation, from a single fertilized cell to a fully developed baby ready for life outside the womb. Tracking your pregnancy week by week gives you a clear picture of what is happening at each stage, when important screenings are due, and what physical changes to expect. Whether this is your first pregnancy or your third, understanding the weekly timeline helps you feel informed and prepared for every milestone ahead.

This guide breaks down all three trimesters with week-by-week developmental highlights, a prenatal testing timeline, weight gain guidelines, and practical examples showing how different pregnancies progress through the same framework.

How Pregnancy Weeks Are Counted

Pregnancy weeks are counted from the first day of your last menstrual period, not from the date of conception. This means that during weeks 1 and 2, you are not actually pregnant yet, as conception typically occurs around the end of week 2 or beginning of week 3 for a standard 28-day cycle. By the time most people learn they are pregnant through a missed period and positive test, they are already considered approximately 4 to 5 weeks along.

This dating convention exists because the last menstrual period provides a consistent, verifiable starting point, while the exact moment of conception is nearly impossible to pinpoint for naturally conceived pregnancies. A full-term pregnancy is 40 weeks (280 days) by this counting method, equivalent to about 38 weeks from actual fertilization.

First Trimester: Weeks 1 Through 13

The first trimester is the most critical period for organ development. By the end of week 13, all major organ systems have formed, and the embryo has officially transitioned to a fetus. This trimester brings the most dramatic hormonal changes for the pregnant individual, often accompanied by nausea, fatigue, and breast tenderness.

Weeks 1 to 4: Conception and Implantation

During weeks 1 and 2, the body prepares for ovulation and the uterine lining thickens. Conception occurs around the end of week 2, when a sperm fertilizes the egg in the fallopian tube. The resulting zygote begins dividing rapidly as it travels toward the uterus over the next several days.

By the end of week 3, the blastocyst implants into the uterine lining, establishing a connection to the maternal blood supply. During week 4, the placenta begins forming and the embryo is organized into three cell layers that will become different organ systems: the ectoderm (skin, nervous system), mesoderm (heart, muscles, bones), and endoderm (lungs, digestive system). At this point, the embryo is roughly the size of a poppy seed.

Weeks 5 to 8: Heartbeat and Organ Formation

Week 5 marks the beginning of the embryonic period, the most intensive phase of organ development. The neural tube, which becomes the brain and spinal cord, begins closing. The heart starts beating around week 5 or 6, visible on ultrasound as a tiny flickering motion. By week 6, arm and leg buds appear.

During weeks 7 and 8, facial features begin forming, including the eyes, nostrils, and mouth. Fingers and toes start to differentiate. The brain is growing rapidly, producing about 100 new neurons per minute. By the end of week 8, the embryo measures about half an inch long and all major organ systems have at least a rudimentary structure in place, making this period especially sensitive to environmental exposures.

Weeks 9 to 13: From Embryo to Fetus

At week 9, the embryo officially becomes a fetus. Bones begin to harden, and reproductive organs start developing, though external genitalia are not yet distinguishable. By week 10, all vital organs are functioning at a basic level, and the risk of major birth defects decreases significantly.

During weeks 11 to 13, the fetus grows rapidly, reaching about 3 inches in length by the end of the first trimester. Fingernails form, the face becomes more defined, and the fetus begins making swallowing and kicking movements, though these are too small to be felt. The first-trimester screening ultrasound, if offered, typically occurs between weeks 11 and 14 and measures the nuchal translucency to assess chromosomal risk.

Second Trimester: Weeks 14 Through 27

Often called the golden trimester, the second trimester brings relief from early pregnancy symptoms for many individuals. Energy returns, nausea typically subsides, and the pregnancy becomes visibly apparent. The fetus undergoes major growth and begins developing recognizable behaviors like sucking, swallowing, and responding to sound.

Weeks 14 to 18: Movement and Growth

By week 14, the fetus is about 3.5 inches long and can make facial expressions. Fine hair called lanugo covers the body, helping regulate temperature. At week 16, the fetus is about 4.5 inches and may begin making coordinated movements. Second-time parents sometimes feel the first flutters of movement, called quickening, as early as week 16, while first-time parents usually notice this between weeks 20 and 25.

Week 18 marks a point where the fetus can hear sounds. The ears have developed enough to detect the parent's heartbeat and voice, and loud external noises may cause a startle response. The fetus now weighs about 7 ounces and is approximately 5.5 inches long.

Weeks 19 to 23: Anatomy Scan and Hearing

The mid-pregnancy anatomy scan is typically scheduled between weeks 18 and 22. This detailed ultrasound examines the brain, spine, heart, kidneys, limbs, and other organs. The placenta location is assessed, and fetal sex can usually be identified if desired. This scan is one of the most anticipated appointments for many families.

By week 20, the fetus is about 6.5 inches long, and a waxy coating called vernix caseosa begins forming to protect the skin from the amniotic fluid. At week 23, the lungs begin producing surfactant, a substance critical for breathing after birth, though lung maturity is still many weeks away. The fetus now weighs about one pound.

Weeks 24 to 27: Viability Milestone

Week 24 is considered the threshold of viability, the earliest point at which a premature baby may survive outside the womb with intensive medical care. Survival rates at 24 weeks are approximately 40% to 70% in well-equipped neonatal intensive care units, and these rates improve significantly with each additional week of gestation.

During weeks 25 to 27, the fetus develops regular sleep and wake cycles, responds to light and sound with increased activity, and begins storing fat under the skin. By the end of the second trimester, the fetus weighs approximately 2 pounds and is about 14 inches long. The glucose tolerance test for gestational diabetes screening is typically performed between weeks 24 and 28.

Third Trimester: Weeks 28 Through 40

The final trimester focuses on growth, fat storage, and organ maturation, especially the lungs and brain. The pregnant individual may experience increased back pain, frequent urination, Braxton Hicks contractions, and difficulty sleeping as the uterus reaches its maximum size. Prenatal visits increase to every two weeks and then weekly as the due date approaches.

Weeks 28 to 32: Rapid Brain Development

The brain undergoes its most rapid growth phase during the third trimester. By week 28, the brain surface begins developing the characteristic folds and grooves that dramatically increase surface area. The fetus can now open and close its eyes, and the pupils respond to light. Thalamic brain connections form, enabling the fetus to experience REM sleep and possibly dream.

At week 32, the fetus weighs about 3.75 pounds and is roughly 17 inches long. The bones are fully formed but still soft and flexible. Fat continues to accumulate under the skin, giving the body a rounder appearance. Most fetuses assume a head-down position by 32 weeks in preparation for birth, though some do not settle into this position until later.

Weeks 33 to 36: Lung Maturation

Lung maturation is the critical developmental milestone of weeks 33 to 36. Surfactant production increases significantly, and by week 36 the lungs are typically mature enough to support breathing without medical assistance. Babies born at 36 weeks still face some risk of respiratory issues but generally do well with minimal intervention.

The Group B streptococcus screening swab is performed between weeks 35 and 37. This test determines whether the pregnant individual carries GBS bacteria, which is harmless to adults but can cause serious infection in newborns during delivery. If the test is positive, intravenous antibiotics are administered during labor to protect the baby.

Weeks 37 to 40: Full Term

At week 37, the pregnancy is classified as early term. The fetus continues adding about half a pound per week, building fat reserves and strengthening immune defenses. By week 39, the pregnancy reaches full term, and the fetus typically weighs between 6 and 9 pounds.

During weeks 38 to 40, the fetus drops lower into the pelvis in a process called lightening or engagement, and the cervix begins softening and thinning in preparation for labor. The lungs produce a final burst of surfactant. The vernix coating is mostly absorbed, and the lanugo has largely shed. The fetus is fully prepared for life outside the womb.

Prenatal Tests Timeline

Practical Pregnancy Tracking Examples

Example 1: First Pregnancy

Amara, 29, discovers she is pregnant after missing her period. Her LMP was March 10, 2026, giving her an estimated due date of December 15, 2026. At her first appointment at 8 weeks, a dating ultrasound confirms the gestational age within 3 days of the LMP-based estimate.

• First trimester ends: June 8, 2026 (week 13)
• Anatomy scan scheduled: July 14, 2026 (week 18)
• Glucose screening: August 25, 2026 (week 24)
• GBS screening: November 3, 2026 (week 35)
• Full term begins: December 1, 2026 (week 39)

Amara experiences morning sickness from weeks 6 through 14, feels the first fetal movements at week 22, and begins weekly visits at week 36. As a first-time parent, she delivers at 40 weeks and 3 days, which is typical for first pregnancies.

Example 2: Second Pregnancy

Carlos and Mei are expecting their second child. Mei's LMP was January 5, 2026, with a due date of October 12, 2026. Having been through pregnancy before, Mei recognizes the early symptoms sooner and schedules her first prenatal visit at 6 weeks.

• First movement felt: Week 17 (earlier than her first pregnancy at week 21)
• Anatomy scan: May 12, 2026, shows normal development
• Glucose screening passed at week 26
• GBS screening negative at week 36

Mei notices that her belly shows earlier in this pregnancy, which is common for subsequent pregnancies as the abdominal muscles have been stretched before. She delivers at 39 weeks and 1 day, consistent with the tendency for second babies to arrive slightly earlier than first babies.

Example 3: High-Risk Pregnancy

Rachel, 38, is pregnant with her first child and has been diagnosed with gestational diabetes at her glucose screening during week 25. Her LMP was February 20, 2026, with a due date of November 27, 2026.

• Additional growth ultrasounds scheduled every 3-4 weeks starting at week 28
• Blood sugar monitoring four times daily after gestational diabetes diagnosis
• Non-stress tests beginning at week 32, twice weekly
• Fetal growth is tracking at the 70th percentile at week 34

Rachel's healthcare team monitors her more closely than a typical pregnancy, with visits every 1 to 2 weeks during the third trimester. Her blood sugar is well controlled with dietary changes, and her provider discusses the plan for delivery timing. Because gestational diabetes slightly increases the risk of complications past the due date, her team recommends induction at 39 weeks if labor has not started naturally.

Weight Gain Guidelines by Trimester

Weight gain recommendations vary based on pre-pregnancy body mass index. The following table reflects guidelines from the Institute of Medicine, which are widely used in prenatal care.

Most weight gain occurs during the second and third trimesters. First-trimester gain is typically minimal, around 1 to 4 pounds total. Weight gain includes not just the baby but also the placenta, amniotic fluid, increased blood volume, breast tissue, and fat stores for breastfeeding. These guidelines are not rigid targets but ranges to discuss with your healthcare provider based on your individual situation.

Tips for Tracking Your Pregnancy Progress

Use a pregnancy tracker app or journal to record your weekly milestones, symptoms, and questions for your healthcare provider. Having this record helps you notice patterns, remember when symptoms started or resolved, and makes prenatal visits more productive when you can reference specific dates.

Attend all scheduled prenatal appointments even when you feel perfectly healthy. Many pregnancy complications, including gestational diabetes, preeclampsia, and fetal growth restrictions, are detected through routine screening before symptoms appear. Early detection allows for interventions that significantly improve outcomes.

Learn the warning signs that require immediate medical attention regardless of gestational week: heavy vaginal bleeding, severe abdominal pain, persistent severe headaches with visual changes, sudden swelling of the face and hands, fluid leaking from the vagina, and a significant decrease in fetal movement after 28 weeks. Knowing these signs empowers you to act quickly when it matters.

Common Mistakes to Avoid

• Comparing your pregnancy to social media timelines. Every pregnancy progresses differently. Belly size, symptom severity, and milestone timing vary enormously between individuals. What matters is that your healthcare provider confirms your pregnancy is progressing normally at each visit.
• Skipping the glucose tolerance test. Gestational diabetes can develop in individuals with no prior risk factors. The test is a simple screening that catches a condition affecting 2% to 10% of pregnancies. Undiagnosed gestational diabetes increases risks for both parent and baby.
• Ignoring reduced fetal movement. After 28 weeks, consistent fetal movement is an important indicator of well-being. If you notice a significant decrease in your baby's usual movement pattern, contact your healthcare provider promptly for evaluation rather than waiting for the next scheduled visit.
• Relying on internet research over your healthcare team. Online pregnancy information varies widely in accuracy and relevance. Use reputable sources to supplement your understanding, but always discuss concerns, test results, and decisions with your prenatal care provider who knows your specific medical history.
• Stressing over exact weekly comparisons. Fetal development descriptions are averages. Your baby may reach milestones a few days earlier or later than the textbook timeline, and this is completely normal. Focus on the overall trajectory confirmed by your prenatal visits rather than matching every single weekly benchmark.

Last updated: February 23, 2026