First Trimester of Pregnancy: Complete Week-by-Week Guide

Weeks 1 Through 4: Conception and Implantation

Pregnancy is dated from the first day of your last menstrual period (LMP), which means that during "weeks 1 and 2" of pregnancy, you are not actually pregnant yet. Ovulation typically occurs around day 14 of a 28-day cycle, and conception happens when a sperm fertilizes the egg, usually in the fallopian tube. The fertilized egg, now called a zygote, begins dividing as it travels down the fallopian tube toward the uterus, a journey that takes about three to four days.

By the time the developing embryo reaches the uterus, it has become a hollow ball of cells called a blastocyst. Around day 6 to 10 after fertilization (approximately week 3 to 4 of pregnancy), the blastocyst implants in the uterine lining. Some women experience light spotting called implantation bleeding at this point, which can be mistaken for a light period. The implanted blastocyst begins producing human chorionic gonadotropin (hCG), the hormone that pregnancy tests detect.

During week 4, the embryo is establishing its basic structure. Three distinct cell layers form that will develop into all the body's tissues and organs: the ectoderm (which will become the skin, nervous system, and eyes), the mesoderm (which will become the heart, bones, muscles, and kidneys), and the endoderm (which will become the lungs, liver, and digestive system). At this stage, the embryo is about the size of a poppy seed. Use our due date calculator to estimate your expected delivery date based on your last menstrual period.

Weeks 5 Through 8: Rapid Development

This is a period of remarkably rapid growth and organ formation. During week 5, the embryo's heart begins to form and will start beating by the end of this week or early in week 6, pumping blood through a rudimentary circulatory system. The neural tube, which will become the brain and spinal cord, is closing. The embryo is about the size of a sesame seed.

By week 6, the embryo has developed tiny buds that will become the arms and legs. Facial features are beginning to form, including small depressions where the eyes will be and openings for the nostrils. The heart is beating at about 100 to 160 beats per minute, which can often be detected on an ultrasound at this stage. This is also when many women first begin to experience morning sickness, as hCG levels are rising rapidly.

During weeks 7 and 8, development accelerates further. Fingers and toes begin to form, though they are still webbed. The brain is growing rapidly, and the embryo starts making spontaneous movements, though these are far too small to feel. The intestines are forming, and the liver begins producing blood cells. Eyelids are forming and will soon fuse shut, not opening again until about week 26. By the end of week 8, all major organs and body systems have begun to form, and the embryo is now called a fetus. At this point, the fetus is about the size of a kidney bean, approximately half an inch long.

Weeks 9 Through 13: The Fetal Period Begins

From week 9 onward, the developing baby is officially referred to as a fetus. This marks a shift from organ formation (organogenesis) to a period of growth and maturation of the structures that have already been established.

During weeks 9 and 10, the fetus's tail disappears, toes and fingers separate, and tooth buds form. External genitalia begin to differentiate, though it is still too early to determine sex by ultrasound. The fetus can now move its limbs, open and close its fists, and even hiccup, though you will not feel any of these movements for several more weeks.

By weeks 11 and 12, the fetus is about 2 inches long and weighs approximately half an ounce. The bones are beginning to harden, fingernails are forming, and the face has a distinctly human appearance. The fetus can swallow amniotic fluid, and the kidneys begin producing urine. For many women, weeks 11 to 12 mark the beginning of relief from morning sickness, as the placenta takes over hormone production from the corpus luteum and hCG levels begin to stabilize.

At week 13, the first trimester comes to a close. The fetus is about 3 inches long and weighs roughly an ounce. All major organs are formed and functioning, and the risk of miscarriage drops significantly. Many parents choose this milestone as the time to share their pregnancy news more widely. You can track your progress with our 12 weeks pregnant guide for more details about this stage.

Common First Trimester Symptoms

The first trimester brings a wide range of physical symptoms driven primarily by rapidly rising levels of hormones, particularly hCG, progesterone, and estrogen. Understanding what is normal can help reduce anxiety and help you manage these symptoms effectively.

Nausea and Vomiting (Morning Sickness)

Despite its name, morning sickness can strike at any time of day or night. It affects approximately 70 to 80 percent of pregnant women and typically begins around week 6, peaks between weeks 8 and 11, and resolves by weeks 12 to 14 for most women. While uncomfortable, morning sickness is generally not harmful to the baby. However, if vomiting is so severe that you cannot keep down food or fluids, are losing weight, or feel faint or dehydrated, contact your healthcare provider promptly, as this may indicate hyperemesis gravidarum, a condition that requires medical treatment.

Fatigue

First-trimester fatigue can be profound and overwhelming. Your body is working extraordinarily hard, producing the placenta, increasing blood volume by nearly 50 percent, and supporting rapid fetal development. Progesterone levels are also rising sharply, and this hormone has a sedating effect. Rest when you can, prioritize sleep, and know that energy typically returns in the second trimester.

Breast Changes

Breast tenderness, swelling, and sensitivity are among the earliest pregnancy symptoms, often noticeable even before a missed period. Your breasts may feel sore, heavy, or tingly. The areolas may darken and enlarge. These changes are driven by hormonal shifts preparing your breasts for eventual milk production. Wearing a supportive bra, including at night, can help manage discomfort.

Frequent Urination

Increased blood flow to the kidneys and the growing uterus pressing on the bladder cause many women to urinate more frequently during the first trimester. This is normal and not a cause for concern unless accompanied by pain, burning, or blood in the urine, which could indicate a urinary tract infection that requires treatment.

Food Aversions and Cravings

Hormonal changes can dramatically alter your sense of taste and smell, leading to strong aversions to foods you previously enjoyed and sudden cravings for specific foods. Common aversions include coffee, meat, and foods with strong odors. While cravings are usually harmless, craving non-food items like dirt or ice (a condition called pica) should be discussed with your healthcare provider, as it may indicate a nutritional deficiency.

Your First Prenatal Visit

Your first prenatal appointment typically occurs between weeks 6 and 8 and is usually the most comprehensive visit of your pregnancy. Being prepared for what to expect can help reduce anxiety and ensure you get the most from this important appointment.

The visit will typically include a detailed medical history review covering your personal health history, family medical history, previous pregnancies, current medications, and lifestyle factors. A physical examination including blood pressure, weight, and often a pelvic exam will be performed. Blood tests will check your blood type and Rh factor, hemoglobin and hematocrit (to screen for anemia), immunity to rubella and varicella, sexually transmitted infection screening, and other routine tests.

An early ultrasound may be performed to confirm the pregnancy is in the uterus (not ectopic), estimate gestational age, detect the heartbeat, and determine whether you are carrying multiples. Your provider will also discuss prenatal vitamins, nutrition guidelines, exercise recommendations, activities and substances to avoid, and a schedule for upcoming appointments and screenings.

Come prepared with questions. Common first-visit questions include asking about what medications are safe during pregnancy, how much weight gain is recommended, what exercise is appropriate, when genetic screening tests are offered, and how to reach your provider for urgent concerns between appointments.

Managing Morning Sickness

While morning sickness cannot always be completely eliminated, several evidence-based strategies can significantly reduce its severity and frequency.

Eat small, frequent meals throughout the day rather than three large meals. An empty stomach tends to worsen nausea, so keeping something in your stomach at all times helps. Many women find that keeping crackers or dry toast by the bed and eating a small amount before getting up in the morning reduces early morning nausea.

Ginger has been shown in multiple clinical studies to be effective for reducing pregnancy-related nausea. Options include ginger tea, ginger ale made with real ginger, ginger candies, or ginger capsules. The effective dose in studies is typically 250 milligrams of ginger four times daily.

Stay hydrated even if you cannot eat much. Sipping water, clear broth, or electrolyte drinks throughout the day is important. If plain water worsens nausea, try adding lemon slices or drinking it very cold. Popsicles and ice chips are another way to maintain hydration.

Avoid trigger foods and smells. Common triggers include greasy, spicy, or strong-smelling foods. Eating cold or room-temperature foods can help because they produce less odor than hot foods. Acupressure wristbands, which apply pressure to the P6 (Nei Guan) point on the inner wrist, have shown moderate effectiveness in some studies. Vitamin B6 (pyridoxine) at 10 to 25 milligrams three times daily has been shown to reduce nausea and is considered safe during pregnancy. If these measures are not sufficient, speak with your healthcare provider about prescription anti-nausea medications that are safe during pregnancy.

Nutrition in the First Trimester

Good nutrition during the first trimester supports the rapid cell division and organ formation that is taking place. While you do not need to eat significantly more calories during the first trimester (caloric needs increase by about 340 calories per day starting in the second trimester), the quality of your diet matters greatly.

Folate (or its synthetic form, folic acid) is the single most critical nutrient in the first trimester. It plays a vital role in neural tube development, and adequate intake significantly reduces the risk of neural tube defects such as spina bifida. The recommended daily intake is 600 micrograms of dietary folate equivalents, and most prenatal vitamins contain 400 to 800 micrograms of folic acid. Food sources include dark leafy greens, fortified cereals, beans, lentils, and citrus fruits.

Iron needs increase during pregnancy to support the expanding blood volume and the developing baby's blood supply. The recommended daily intake is 27 milligrams, nearly double the non-pregnant recommendation. Good sources include lean red meat, poultry, fish, beans, fortified cereals, and dark leafy greens. Iron absorption is enhanced by vitamin C, so pairing iron-rich foods with citrus fruits or other vitamin C sources is beneficial.

Calcium is essential for the development of the baby's bones, teeth, heart, muscles, and nervous system. Aim for 1,000 milligrams daily from dairy products, fortified plant milks, sardines, tofu, and leafy greens. DHA (docosahexaenoic acid), an omega-3 fatty acid found in fatty fish, is important for fetal brain and eye development. If you do not regularly eat fish, consider a DHA supplement. Protein needs also increase during pregnancy, and you should aim for at least 71 grams daily from varied sources.

What to Avoid During the First Trimester

The first trimester is the period of greatest vulnerability for the developing embryo, as major organ systems are forming. Avoiding harmful substances and exposures during this time is critically important.

Alcohol should be completely avoided. There is no known safe amount of alcohol during pregnancy. Alcohol crosses the placenta and can interfere with fetal development at any stage, potentially causing fetal alcohol spectrum disorders. Smoking and secondhand smoke exposure should also be eliminated, as they increase the risk of miscarriage, preterm birth, low birth weight, and other complications.

Certain foods pose specific risks during pregnancy. Raw or undercooked meat, poultry, seafood, and eggs can carry bacteria like Salmonella, E. coli, and Toxoplasma. Unpasteurized dairy products and soft cheeses can contain Listeria, which is particularly dangerous during pregnancy. High-mercury fish such as shark, swordfish, king mackerel, and tilefish should be avoided, though low-mercury fish like salmon, shrimp, and tilapia are safe and beneficial in moderation. Caffeine should be limited to 200 milligrams per day, equivalent to roughly one 12-ounce cup of coffee.

Avoid cat litter (which can carry Toxoplasma), hot tubs and saunas (excessive heat can be harmful to the developing baby, particularly in early pregnancy), certain medications (always check with your provider before taking any medication, including over-the-counter products), and exposure to pesticides, lead, and other environmental toxins.

Emotional Changes in the First Trimester

The first trimester is not just a physical experience; it is an intensely emotional one as well. Hormonal surges, physical symptoms, and the enormity of impending parenthood combine to create a complex emotional landscape that is completely normal.

Mood swings are extremely common, driven partly by hormonal fluctuations and partly by the adjustment to a new reality. You may feel joyful one moment and anxious or tearful the next. Worry about the pregnancy's viability, the baby's health, and your ability to be a good parent are nearly universal experiences, particularly in the first trimester when the risk of miscarriage is highest.

Some women experience ambivalence about the pregnancy even when it was planned and wanted. This is normal and does not mean you will not be a good parent. The gap between the abstract idea of having a baby and the physical reality of being pregnant can take time to process. Give yourself grace and patience during this adjustment period.

If you are experiencing persistent sadness, hopelessness, excessive anxiety, inability to function in daily life, or thoughts of self-harm, talk to your healthcare provider. Prenatal depression and anxiety are real conditions that affect up to 15 to 20 percent of pregnant women and are treatable. You do not need to struggle through these feelings alone.

When to Call Your Doctor

While many first-trimester symptoms are normal and expected, certain signs warrant prompt medical attention. Contact your healthcare provider immediately if you experience any of the following: heavy bleeding (soaking a pad in an hour or passing large clots), severe or persistent abdominal or pelvic pain, fever above 100.4 degrees Fahrenheit, severe vomiting that prevents you from keeping down any food or liquid for 24 hours, painful or burning urination, dizziness or fainting, or severe headache with visual changes.

You should also contact your provider if you experience vaginal discharge that is foul-smelling, green, or yellow; intense itching; or exposure to someone with an active infection such as chickenpox, rubella, or COVID-19. When in doubt, it is always better to call and ask than to wait and worry. Your healthcare team would rather receive a call about something that turns out to be normal than miss an opportunity to address a genuine concern.