Pregnancy lasts 40 weeks, divided into 3 trimesters. Your baby grows from a single fertilized cell to a full-term newborn averaging 7.5 pounds and 20 inches long. Every week brings measurable changes to both mother and baby.
What to Expect Each Trimester at a Glance
Pregnancy is divided into three trimesters spanning 40 weeks, each with distinct developmental phases and medical priorities.
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First | 1–12 | Implantation, heartbeat, organ formation |
| Second | 13–26 | Movement felt, anatomy scan, rapid growth |
| Third | 27–40 | Lung maturity, brain development, birth position |
First Trimester: Weeks 1–12
Weeks 1–4: Conception and Implantation
Weeks 1 and 2 are technically counted before conception, based on the first day of your last menstrual period (LMP), which is the standard medical start point for pregnancy dating. Fertilization typically occurs around Week 2. By Week 4, the fertilized egg implants in the uterine lining and the embryo is about 0.04 inches long.
The placenta begins forming at implantation. It will serve as the baby’s life-support system for the entire pregnancy, delivering oxygen and nutrients while removing waste products.
Weeks 5–6: The Heart Begins to Beat
By Week 5, the embryo is roughly the size of a sesame seed at 0.13 inches. The neural tube, which will become the brain and spinal cord, begins closing during this week. A missed period typically prompts a home pregnancy test, which can detect the hormone hCG (human chorionic gonadotropin, the hormone produced after implantation) as early as Week 4.
At Week 6, a heartbeat is detectable by transvaginal ultrasound. The heart beats at approximately 90–110 beats per minute at this stage, increasing to 140–170 bpm in the weeks that follow.
Weeks 7–8: Facial Features and Limb Buds
By Week 7, the embryo measures about 0.5 inches and has distinct arm and leg buds. The brain is developing rapidly, producing approximately 100 new neurons per minute. The eyes, nose, and mouth begin forming as visible structures.
At Week 8, all major organs are present in rudimentary form. Fingers and toes are beginning to separate. The embryo officially becomes a fetus at the end of Week 10, marking the transition from the embryonic period to the fetal period.
Weeks 9–10: Movement Begins Internally
By Week 9, the fetus measures about 0.9 inches and weighs less than 0.1 ounce. Spontaneous movement begins, although you will not feel it yet. The eyelids form and fuse shut, protecting the developing eyes.
At Week 10, fingers and toes are fully separated and nails begin to grow. The heart has divided into 4 chambers and beats at approximately 160–170 bpm. External genitalia are present but not yet distinguishable by ultrasound.
Weeks 11–12: End of the First Trimester
At Week 11, the fetus is about 1.6 inches long and weighs roughly 0.25 ounce. Tooth buds are forming beneath the gums. The kidneys are producing small amounts of urine, which becomes part of the amniotic fluid.
By Week 12, the fetus measures approximately 2.1 inches and weighs about 0.5 ounce. The nuchal translucency screening (an ultrasound that measures fluid at the back of the neck to screen for chromosomal conditions like Down syndrome) is typically performed between Weeks 11 and 13. Miscarriage risk drops significantly after Week 12.
Second Trimester: Weeks 13–26
Weeks 13–14: Fingerprints and Reflexes
The second trimester begins at Week 13, when the fetus is about 2.9 inches long and weighs roughly 0.8 ounce. Unique fingerprints are forming on the fingertips. Facial muscles allow for primitive expressions including squinting and frowning.
At Week 14, the fetus can make sucking and swallowing motions. Lanugo (fine, downy hair that covers the body and helps regulate temperature in the womb) begins to grow across the entire body. Sex can sometimes be determined by ultrasound at this stage.
Weeks 15–16: Rapid Growth Phase
At Week 15, the fetus measures about 4 inches and weighs approximately 2.5 ounces. Bones are hardening in a process called ossification. The eyes, though fused shut, are sensitive to light and will move away from a bright light source shone at the belly.
By Week 16, the fetus is about 4.6 inches and weighs roughly 3.5 ounces. The maternal-fetal anatomy is increasingly visible on ultrasound. Some women, especially those who have been pregnant before, may begin to feel faint fluttering movements called quickening at this point.
Weeks 17–18: Fat Accumulates
At Week 17, the fetus weighs approximately 5 ounces and measures about 5.1 inches. A protective coating called vernix caseosa (a waxy, cheese-like substance that shields the skin from amniotic fluid) begins forming. The skeleton, previously cartilage, is hardening into bone.
Week 18 brings increased fetal activity. Most first-time mothers begin feeling movement between Weeks 18 and 20. The fetus has developed a sleep and wake cycle, often most active when the mother is at rest.
Weeks 19–20: Anatomy Scan Milestone
The mid-pregnancy anatomy scan (also called the level 2 ultrasound, a detailed imaging exam that checks all major organ systems and fetal growth) is typically scheduled between Weeks 18 and 22, most commonly at Week 20. This scan can identify structural abnormalities, confirm placental position, and reveal fetal sex if desired.
At Week 20, you are exactly halfway through pregnancy. The fetus is about 6.5 inches from crown to rump (roughly 10 inches from head to toe) and weighs around 10.5 ounces.
Weeks 21–22: Viability Approaches
By Week 21, the fetus weighs about 12.7 ounces and measures approximately 10.5 inches head to toe. The digestive system is practicing with swallowed amniotic fluid. Eyebrows and lashes are visible.
Week 22 is considered by many medical centers the threshold of viability (the point at which a premature baby has a reasonable chance of survival with intensive medical care). Survival rates at 22 weeks with specialized NICU care range from 10–35%, improving substantially with each additional week.
Weeks 23–24: Lung Development Begins
At Week 23, the fetus weighs approximately 1.1 pounds. The inner ear is fully developed, and the baby can hear sounds from outside the womb. Brain activity increases significantly during this period.
By Week 24, survival rates with intensive care reach approximately 40–70%. The lungs begin producing surfactant (a substance that coats the inside of the air sacs, preventing them from collapsing after birth). This week is often considered the standard threshold of viability under U.S. medical guidelines.
Weeks 25–26: Brain Grooves Form
At Week 25, the fetus weighs about 1.5 pounds and measures roughly 13.6 inches. The brain surface begins forming the characteristic grooves and ridges that increase surface area and cognitive capacity. Response to sound is now measurable through fetal heart rate changes.
By Week 26, the eyes open for the first time. The lungs continue maturing and the fetus breathes amniotic fluid in practice for breathing air. Survival rates at this stage approach 80% with appropriate NICU support.
Third Trimester: Weeks 27–40
Weeks 27–28: Third Trimester Begins
The third trimester opens at Week 27, when the fetus weighs about 2 pounds and measures approximately 14.4 inches. The brain undergoes dramatic development, with billions of neurons forming connections. The immune system begins developing its own antibodies.
At Week 28, the standard prenatal glucose challenge test (a screening for gestational diabetes, a form of diabetes that develops during pregnancy and affects blood sugar regulation) is typically performed. Fetal movement should be felt consistently at this stage, and kick counts (tracking fetal movement to assess well-being) become a daily monitoring practice.
Weeks 29–30: Bone Marrow Takes Over
At Week 29, the fetus weighs approximately 2.5 pounds. The bone marrow takes over blood cell production from the liver and spleen. The adrenal glands produce hormones that will trigger labor weeks later.
By Week 30, the fetus weighs about 3 pounds and measures roughly 15.7 inches. The pupils can now dilate and constrict in response to light. Brain wave activity shows sleep cycles similar to those observed after birth, including REM sleep.
Weeks 31–32: Rapid Weight Gain
Between Weeks 31 and 37, the fetus gains approximately 0.5 pounds per week. At Week 31, the fetus weighs about 3.3 pounds. The lungs continue maturing and the trachea and diaphragm are fully formed.
At Week 32, most babies settle into a head-down position (vertex presentation) in preparation for birth. Babies born at 32 weeks have a survival rate exceeding 95% with NICU care, and most have very good long-term outcomes.
Weeks 33–34: Immune System Strengthens
The fetus at Week 33 weighs approximately 4.2 pounds and measures about 17.2 inches. Antibodies are transferred from mother to baby through the placenta, providing early immune protection. Bones are fully developed but still soft enough to allow passage through the birth canal.
By Week 34, the fetus weighs around 4.7 pounds. Vernix caseosa thickens. Most organs are fully functional, and babies born at this stage generally do very well, though a NICU stay of a few weeks is common.
Weeks 35–36: Final Preparations
At Week 35, the fetus weighs approximately 5.3 pounds and fat accumulates rapidly beneath the skin, smoothing out the wrinkled appearance. The brain at 35 weeks weighs only about two-thirds of what it will weigh at 40 weeks, making these final weeks critical for neurological development.
By Week 36, the fetus weighs about 5.8 pounds and most organ systems are mature. Late preterm birth (delivery between 34 and 36 weeks, 6 days) accounts for approximately 70% of all preterm births in the United States. Babies born in this window generally do well but may need extra support for feeding and temperature regulation.
Weeks 37–38: Full Term
A pregnancy is considered full term at 37 weeks. The fetus at this point weighs approximately 6.3 pounds and the lungs are mature enough to support breathing without medical assistance in most cases.
At Week 38, the fetus weighs about 6.8 pounds. The average first-time mother delivers at 40 weeks and 5 days, while mothers who have given birth before average 40 weeks and 3 days, according to U.S. birth data.
Weeks 39–40: Ready for Birth
At Week 39, the fetus weighs approximately 7.1 pounds and has built up the fat stores needed for temperature regulation after birth. The skull bones remain slightly separated at the sutures (flexible joints that allow the head to compress during delivery), which assists passage through the birth canal.
At Week 40, the average birth weight is 7.5 pounds and average length is 20 inches. If labor has not begun by 41 weeks, most U.S. providers recommend discussing induction of labor (a medical process to initiate contractions before they begin spontaneously) to reduce risks associated with post-term pregnancy.
Key Prenatal Appointments and Tests by Week
Most low-risk pregnancies require 8 to 14 prenatal visits, with specific screenings tied to gestational age windows.
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| Week | Test or Appointment | Purpose |
|---|---|---|
| 8–10 | First prenatal visit | Confirm pregnancy, establish due date |
| 10–13 | Cell-free DNA / NIPT | Screen for chromosomal conditions |
| 11–13 | Nuchal translucency ultrasound | Screen for Down syndrome |
| 15–20 | Quad screen blood test | Screen for neural tube defects and chromosomal issues |
| 18–22 | Anatomy ultrasound | Check organ development, confirm sex |
| 24–28 | Glucose challenge test | Screen for gestational diabetes |
| 28 | Rh factor / antibody screen | Identify Rh incompatibility |
| 35–37 | Group B Strep (GBS) test | Screen for bacteria that can affect newborn |
| 36+ | Weekly visits begin | Monitor for labor readiness |
Fetal Size by Trimester: A Quick Reference
Fetal length and weight increase dramatically each trimester, from under 1 ounce at week 8 to an average of 7.5 pounds at week 40.
| Week | Length (inches) | Weight |
|---|---|---|
| 8 | 0.6 | 0.04 oz |
| 12 | 2.1 | 0.5 oz |
| 16 | 4.6 | 3.5 oz |
| 20 | 10.0 | 10.6 oz |
| 24 | 11.8 | 1.3 lbs |
| 28 | 14.8 | 2.2 lbs |
| 32 | 16.7 | 3.8 lbs |
| 36 | 18.7 | 5.8 lbs |
| 40 | 20.0 | 7.5 lbs |
Common Symptoms by Trimester
First Trimester Symptoms
The most common first trimester symptoms include nausea (affecting up to 80% of pregnant women), fatigue, breast tenderness, and frequent urination. These symptoms are driven primarily by rising hCG and progesterone levels. Nausea peaks around Weeks 8–10 and typically resolves by Week 14 for most women.
Second Trimester Symptoms
Most women describe the second trimester as the most comfortable phase of pregnancy. Energy returns, nausea typically subsides, and the belly becomes visibly rounded. Round ligament pain (sharp or achy discomfort in the lower abdomen or groin caused by the stretching of ligaments supporting the uterus) is common from Week 14 onward.
Third Trimester Symptoms
Third trimester symptoms reflect the baby’s increasing size. Heartburn, back pain, swollen ankles, and shortness of breath are common. Braxton Hicks contractions (irregular, painless tightening of the uterus that serve as practice contractions and are not a sign of labor) typically become noticeable from Week 28 onward.
Warning Signs That Require Immediate Medical Attention
The following symptoms at any stage of pregnancy warrant an immediate call to your provider or a visit to the emergency room.
- Vaginal bleeding beyond light spotting
- Severe abdominal pain or cramping
- Fever above 100.4 degrees Fahrenheit
- Sudden swelling of the face, hands, or feet
- Severe headache that does not resolve
- Visual disturbances including flashing lights or blurriness
- Decreased fetal movement after Week 28
- Signs of preterm labor before 37 weeks (regular contractions, pelvic pressure, or fluid leaking)
Frequently Asked Questions
When does the baby’s heart start beating?
A fetal heartbeat is typically detectable by transvaginal ultrasound at approximately Week 6 of pregnancy. At this stage, the heart beats at around 90–110 beats per minute, increasing to a peak of 140–170 bpm in the first trimester. By the second trimester, the rate stabilizes to around 120–160 bpm.
When do you start showing during pregnancy?
Most first-time mothers begin showing a visible baby bump between Weeks 16 and 20. Women who have been pregnant before often show earlier, sometimes as early as Week 12–14, because the abdominal muscles have already been stretched. Body type and uterine position also influence when a bump becomes noticeable.
When can you feel the baby move?
Most women feel fetal movement for the first time between Weeks 18 and 25, a sensation called quickening. First-time mothers tend to notice it closer to Week 20–22, while experienced mothers may feel it earlier at Week 16–18 because they recognize the sensation. Movement becomes stronger and more regular through the second and third trimesters.
What week is considered full term?
A pregnancy is considered full term at 37 weeks. However, babies born between 39 and 40 weeks (early full term) have the best outcomes compared to those born earlier in the full-term window. The American College of Obstetricians and Gynecologists recommends avoiding elective delivery before 39 weeks when possible.
When is the anatomy scan done?
The anatomy scan (also called the 20-week ultrasound or level 2 ultrasound) is typically scheduled between Weeks 18 and 22, most commonly at Week 20. This scan examines all major fetal organs and structures, checks the placenta and amniotic fluid levels, and can reveal fetal sex if the parents wish to know.
What week is the baby fully developed?
All major organ systems are present from Week 12, but full maturity, especially of the lungs and brain, is not reached until Weeks 37–40. The lungs are typically mature enough for independent breathing around Week 37, while brain development continues actively through the final weeks and well after birth.
How many weeks is a normal pregnancy?
A normal pregnancy lasts approximately 40 weeks from the first day of the last menstrual period, or 38 weeks from conception. Full term is defined as 37–42 weeks. Pregnancies ending before 37 weeks are considered preterm, and those extending beyond 42 weeks are considered post-term.
When should I go to the hospital for labor?
For first-time mothers, the general guideline is the 5-1-1 rule: contractions that are 5 minutes apart, lasting at least 1 minute each, for at least 1 hour. You should also go immediately if your water breaks, if you experience heavy bleeding, or if fetal movement has significantly decreased. Your provider may give you different instructions based on your specific situation and distance from the hospital.
What is the survival rate for premature babies?
Survival rates depend heavily on gestational age. Babies born at 22 weeks have a roughly 10–35% survival rate with intensive care. By 24 weeks, survival approaches 40–70%, and by 28 weeks it exceeds 90%. Babies born at 32 weeks or later have a survival rate above 95% and generally do very well with appropriate support.
How much weight should I gain during pregnancy?
Weight gain recommendations depend on pre-pregnancy BMI (Body Mass Index, a measure of body fat based on height and weight). Women with a normal BMI (18.5–24.9) are advised to gain 25–35 pounds total. Underweight women should gain 28–40 pounds, overweight women 15–25 pounds, and women with obesity 11–20 pounds, according to current guidelines from the Institute of Medicine.