Pregnancy Week by Week: What to Expect Each Trimester

How Your Due Date Is Calculated

Pregnancy is measured from the first day of your last menstrual period (LMP), even though conception typically occurs about two weeks later during ovulation. The standard method, known as Naegele's rule, adds 280 days (40 weeks) to the first day of your LMP to estimate your due date.

For example, if your last period started on January 1, your estimated due date would be October 8. You can calculate this quickly with our pregnancy due date calculator.

Keep in mind that a due date is an estimate, not a guarantee. Only about 5% of babies arrive on their exact due date. Most deliveries happen within a window of two weeks before to two weeks after the estimated date, and this is considered perfectly normal. An early first-trimester ultrasound, typically performed between weeks 8 and 12, can refine your due date based on the embryo's crown-to-rump length measurement, which is often more accurate than LMP dating.

If you are trying to conceive or want to understand your fertility window, our ovulation calculator can help you identify your most fertile days based on your cycle length.

First Trimester: Weeks 1 Through 12

The first trimester begins on the first day of your last menstrual period and runs through the end of week 12. This is the period of the most dramatic early development, when your baby goes from a fertilized egg to a recognizable human form with a beating heart and developing organs.

Weeks 1-4: Conception and Implantation

During weeks 1 and 2, your body is preparing for ovulation. Conception typically occurs around week 2 or 3 when a sperm fertilizes the egg in the fallopian tube. The fertilized egg, now called a zygote, travels to the uterus and implants in the uterine wall around week 4. At this point, the embryo is smaller than a poppy seed, but the placenta is beginning to form. A home pregnancy test can typically detect the pregnancy hormone hCG around the time of your missed period, roughly week 4 or 5.

Weeks 5-8: Organ Formation Begins

This is a critical period of development. By week 5, the neural tube (which becomes the brain and spinal cord) is forming. By week 6, the heart begins beating, and tiny limb buds appear. By week 8, all major organs have begun to form, facial features are developing, and the embryo is about the size of a raspberry. This is when the embryo officially becomes a fetus.

During these weeks, many women experience the hallmark symptoms of early pregnancy: nausea (morning sickness, which can occur at any time of day), extreme fatigue, breast tenderness and swelling, frequent urination, food aversions or cravings, and heightened sense of smell. Not every woman experiences all of these symptoms, and the intensity varies widely.

Weeks 9-12: Rapid Growth

By week 9, the fetus has fingers and toes. By week 10, vital organs are functioning. By week 12, the fetus is about 2.5 inches long and weighs half an ounce. Bones are beginning to harden, and the fetus can make sucking motions. The risk of miscarriage drops significantly after week 12, which is why many people choose to announce their pregnancy at the end of the first trimester.

First Trimester Prenatal Appointments

Your first prenatal visit typically occurs between weeks 8 and 10. This comprehensive appointment includes a physical exam, blood work (blood type, Rh factor, complete blood count, immunity to rubella and hepatitis B, STI screening), urine tests, and often a dating ultrasound. Between weeks 11 and 13, your provider may offer first-trimester screening, which combines a blood test and nuchal translucency ultrasound to assess the risk of certain chromosomal conditions like Down syndrome. Non-invasive prenatal testing (NIPT), a blood test that screens for chromosomal abnormalities with high accuracy, may also be offered during this period.

Second Trimester: Weeks 13 Through 27

The second trimester is often called the "golden trimester" because many of the uncomfortable symptoms of early pregnancy ease, energy levels improve, and the pregnancy becomes more visible and exciting. This is the period when most women feel their best.

Weeks 13-16: Energy Returns

Nausea typically subsides by weeks 13 to 14 for most women. Your energy level increases significantly compared to the exhaustion of the first trimester. The fetus is about 3 to 4 inches long and can make facial expressions. By week 14, the sex organs have developed enough that sex may be visible on ultrasound, though the anatomy scan at week 18-22 is the standard time for sex determination. You may notice a small baby bump appearing, especially in subsequent pregnancies.

Weeks 17-20: Feeling Movement

Between weeks 18 and 25, most first-time mothers feel their baby move for the first time. This sensation, called quickening, is often described as fluttering, bubbling, or a light tapping. Women who have been pregnant before may recognize these movements as early as week 13 to 16. The fetus is now about 6 inches long and weighs around 10 ounces. Hearing is developing, and the baby can respond to sound.

Weeks 18-22: The Anatomy Scan

The mid-pregnancy ultrasound, usually performed between weeks 18 and 22, is one of the most detailed assessments of your baby. The sonographer measures the head, abdomen, and femur; checks the brain, heart, kidneys, spine, and other organs; evaluates the placenta position and amniotic fluid level; and can determine the baby's sex if you want to know. This scan can also detect certain structural abnormalities, giving you and your doctor important information for the remainder of the pregnancy.

Weeks 21-27: Rapid Growth and Viability

During the second half of this trimester, the baby gains weight rapidly. By week 24, the lungs begin producing surfactant, a substance necessary for breathing outside the womb. Week 24 is considered the threshold of viability, meaning that with intensive medical care, babies born at this stage have a chance of survival, though outcomes improve significantly with each additional week in the womb. By week 27, the baby weighs about 2 pounds, can open and close eyes, and has regular sleep-wake cycles.

Common symptoms during the second trimester include back pain as your center of gravity shifts, round ligament pain (sharp twinges in the lower abdomen), skin changes including the linea nigra (dark line on the abdomen), nasal congestion, and mild swelling in the hands and feet. Most of these are normal and manageable.

Second Trimester Prenatal Tests

Between weeks 24 and 28, you will have a glucose screening test to check for gestational diabetes. This involves drinking a glucose solution and having your blood drawn one hour later. If the initial screening is elevated, a follow-up three-hour glucose tolerance test is performed. Gestational diabetes affects about 6-9% of pregnancies and is manageable with diet, exercise, and sometimes medication.

Third Trimester: Weeks 28 Through 40

The third trimester is the final stretch. Your baby is gaining weight rapidly, your body is preparing for labor, and prenatal visits become more frequent. This trimester can feel physically demanding, but every week brings your baby closer to full-term readiness.

Weeks 28-32: Baby Gains Weight

The baby's brain is developing rapidly, bones are continuing to harden (except for the skull, which remains soft for delivery), and body fat is accumulating. By week 28, the baby weighs about 2.5 pounds. By week 32, the baby is about 4 pounds and 16 to 17 inches long. The baby practices breathing by inhaling amniotic fluid and has distinct periods of sleep and wakefulness.

You may experience Braxton Hicks contractions, which are irregular tightening of the uterus that can feel like mild cramping. These practice contractions are normal and help your uterus prepare for labor. They differ from true labor contractions in that they are irregular, do not increase in intensity, and stop when you change position or rest.

Weeks 33-36: Positioning for Birth

During these weeks, most babies turn into a head-down position in preparation for delivery. Your doctor will check the baby's position at your appointments. If the baby is still breech (feet or buttocks down) at week 36, your provider may discuss options including external cephalic version (a procedure to manually turn the baby) or planned cesarean delivery.

You will likely begin seeing your provider every two weeks, then weekly starting at week 36. Between weeks 36 and 37, a Group B streptococcus (GBS) test is performed using a vaginal and rectal swab. About 25% of women carry GBS bacteria, which is harmless to you but can be dangerous to the baby during delivery. If you test positive, you will receive antibiotics during labor to protect the baby.

Weeks 37-40: Full Term

A baby is considered early term at 37 weeks, full term at 39 weeks, and late term at 41 weeks. At 37 weeks, the baby weighs approximately 6 to 6.5 pounds and is about 19 inches long. By 40 weeks, the average baby weighs 7 to 8 pounds and measures 19 to 21 inches. The lungs and brain continue maturing through week 39, which is why elective deliveries before 39 weeks are generally discouraged unless medically necessary.

In the final weeks, you may experience increased pelvic pressure as the baby drops lower (called lightening or engagement), more frequent Braxton Hicks contractions, difficulty sleeping, increased urination as the baby presses on your bladder, and a burst of energy sometimes called nesting. Signs that labor may be approaching include loss of the mucus plug, a bloody show, regular contractions that increase in frequency and intensity, and rupture of membranes (water breaking).

Third Trimester Symptoms

Common third-trimester symptoms include back pain, heartburn, swelling in the feet and ankles, shortness of breath, insomnia, hemorrhoids, and fatigue. While these symptoms are normal, contact your doctor immediately if you experience severe headaches, vision changes, sudden or severe swelling, decreased fetal movement, vaginal bleeding, or persistent abdominal pain. These could be signs of complications like preeclampsia that require prompt medical attention.

Key Prenatal Appointments Timeline

Most providers follow a schedule of monthly visits through week 28, biweekly visits from week 28 to 36, and weekly visits from week 36 until delivery. If your pregnancy is high-risk, you may have more frequent visits and additional monitoring.

Nutrition Needs During Pregnancy

Good nutrition supports your baby's development and your own health throughout pregnancy. Here are the key nutrients and how much you need:

Calories

Contrary to the popular saying, you are not eating for two. During the first trimester, you do not need any extra calories. In the second trimester, add approximately 340 extra calories per day. In the third trimester, add about 450 extra calories per day. Total calorie needs vary based on your pre-pregnancy weight, activity level, and whether you are carrying multiples.

Essential Nutrients

• Folic acid (600 mcg/day): Critical for neural tube development, especially in the first trimester. Start a prenatal vitamin with folic acid before conception if possible. Sources include leafy greens, fortified cereals, beans, and citrus fruits.
• Iron (27 mg/day): Supports the increased blood volume needed during pregnancy and prevents anemia. Sources include lean red meat, poultry, fish, beans, spinach, and fortified cereals. Iron absorption improves when consumed with vitamin C.
• Calcium (1,000 mg/day): Necessary for your baby's bone and tooth development. If you do not consume enough calcium, your body will pull it from your own bones. Sources include dairy products, fortified plant milks, leafy greens, and tofu.
• DHA omega-3 (200-300 mg/day): Supports brain and eye development. Sources include fatty fish (salmon, sardines), algae-based supplements, and fortified eggs. Choose low-mercury fish options.
• Vitamin D (600 IU/day): Works with calcium for bone development and supports immune function. Sources include sunlight, fortified milk, fatty fish, and supplements.

Foods to Avoid

Certain foods carry a higher risk of bacteria, parasites, or toxins that can harm your developing baby:

• Raw or undercooked fish, meat, and eggs
• Unpasteurized dairy products and juices
• Deli meats and hot dogs (unless heated to steaming)
• High-mercury fish (swordfish, shark, king mackerel, tilefish)
• Alcohol (no amount has been proven safe during pregnancy)
• Excessive caffeine (limit to 200 mg per day, roughly one 12-ounce cup of coffee)

When to Call Your Doctor

While most pregnancy symptoms are normal, certain signs require immediate medical attention. Contact your healthcare provider right away if you experience:

• Vaginal bleeding at any stage of pregnancy
• Severe or persistent abdominal pain or cramping
• Severe headaches that do not improve with rest and hydration
• Vision changes such as blurriness, seeing spots, or light sensitivity
• Sudden or severe swelling of the face, hands, or legs
• A noticeable decrease in fetal movement after week 28
• Fluid leaking from the vagina
• Fever above 100.4 degrees Fahrenheit
• Painful or burning urination (possible urinary tract infection)
• Regular contractions before 37 weeks

Trust your instincts. If something feels wrong, it is always better to call your provider and be reassured than to wait and risk a complication.

The Bottom Line

Pregnancy is a complex and beautiful process that unfolds over 40 weeks of remarkable development. Each trimester brings its own challenges, milestones, and moments of wonder. By understanding what is happening at each stage, keeping up with prenatal appointments, eating well, and knowing when to seek help, you are giving yourself and your baby the best possible start.

Every pregnancy is different. Your experience may not match anyone else's timeline perfectly, and that is completely normal. Work closely with your healthcare provider, ask questions whenever you have them, and remember that the vast majority of pregnancies result in healthy babies and healthy mothers. Use our pregnancy due date calculator to track where you are in your journey.

Frequently Asked Questions