Pregnancy Week by Week: Complete Guide to Stages & Development

What Is Pregnancy and How Does It Begin?

Pregnancy begins when a sperm fertilizes an egg, creating a single cell called a zygote that implants in the uterine wall and develops into an embryo and then a fetus over approximately 40 weeks. The journey from conception to birth involves remarkable cellular division, organ formation, and growth.

Pregnancy is a complex biological process that transforms a single fertilized cell into a fully developed human being capable of surviving outside the womb. This extraordinary journey typically lasts about 40 weeks from the first day of the last menstrual period (LMP), or approximately 38 weeks from the actual moment of conception. Understanding this timeline is crucial because healthcare providers use the LMP date to calculate the estimated due date and track fetal development milestones.

The process begins in the fallopian tube, where a sperm cell penetrates the outer layer of an egg cell released during ovulation. This fertilization creates a zygote containing genetic material from both parents, establishing the blueprint for a unique individual. Within hours, this single cell begins dividing, first into two cells, then four, then eight, continuing this process as it travels down the fallopian tube toward the uterus over the course of several days.

By the time the fertilized egg reaches the uterus, approximately five to six days after fertilization, it has developed into a blastocyst—a hollow ball of about 100 cells with an inner cell mass that will become the embryo and an outer layer that will form the placenta. Implantation occurs when this blastocyst burrows into the thickened uterine lining, a process that takes place between days 6 and 12 after fertilization. Some women experience light spotting during implantation, which can sometimes be mistaken for a light period.

Once implantation is complete, the body begins producing human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests. This hormone signals the ovaries to continue producing progesterone, which maintains the uterine lining and prevents menstruation. The developing placenta takes over hormone production around weeks 10-12, establishing the crucial connection between mother and developing baby that will sustain the pregnancy.

The Three Trimesters of Pregnancy

Medical professionals divide pregnancy into three trimesters, each lasting approximately 13-14 weeks. This division helps track developmental milestones and anticipate the changes that occur at different stages. The first trimester (weeks 1-12) focuses on foundational development—the formation of all major organ systems. The second trimester (weeks 13-27) is characterized by rapid growth and the development of more refined features. The third trimester (weeks 28-40) involves final maturation and preparation for life outside the womb.

Each trimester brings distinct experiences for the pregnant person, from the early symptoms of nausea and fatigue to the later sensations of fetal movement and the physical demands of carrying a nearly full-term baby. Understanding these phases helps expectant parents prepare emotionally and practically for what lies ahead while knowing when to seek medical guidance.

What Happens During the First Trimester?

The first trimester (weeks 1-12) is the most critical period for fetal development, when all major organs and body systems begin to form. During this time, the embryo grows from a single cell to about 3 inches long, with a recognizable human form, beating heart, and functioning nervous system.

The first trimester represents the most transformative period of pregnancy, during which a microscopic cluster of cells develops into a recognizable miniature human being. This period carries the highest risk of miscarriage, with most pregnancy losses occurring before week 12. The rapid cellular division and organ formation that occur during these early weeks make the developing embryo particularly vulnerable to environmental factors, which is why avoiding harmful substances and taking prenatal vitamins is especially important during this time.

During weeks 1-4, most women don't yet know they're pregnant. Fertilization typically occurs around week 2, and implantation happens around week 4. By the end of week 4, the embryo is about the size of a poppy seed and consists of two layers of cells that will develop into all the body's organs and tissues. The amniotic sac begins to form, and the placenta starts developing to provide nutrients and oxygen.

Weeks 5-8 bring dramatic changes as the embryo transforms from a flat disk into a curved, elongated shape. The neural tube, which will become the brain and spinal cord, forms during week 5—this is why adequate folic acid intake before and during early pregnancy is crucial for preventing neural tube defects like spina bifida. The heart begins beating around day 22, though it won't be detectable on ultrasound until about week 6. Arm and leg buds appear, and facial features begin to develop, including the eyes, ears, and mouth.

By weeks 9-12, the embryo is now called a fetus. All major organs have begun forming, and the risk of birth defects from external exposures decreases significantly. The fetus can now move, though movements are too small to feel. Fingers and toes become distinct, and the reproductive organs begin developing. By week 12, the fetus is about 2.5-3 inches long and weighs approximately half an ounce. The first trimester screening, which includes blood tests and an ultrasound, can assess the risk of chromosomal abnormalities.

Common First Trimester Symptoms

The first trimester often brings challenging physical symptoms as the body adjusts to pregnancy hormones. Morning sickness, despite its name, can occur at any time of day and affects up to 80% of pregnant women to some degree. Severe nausea and vomiting (hyperemesis gravidarum) requiring medical treatment affects about 1-3% of pregnancies. Other common symptoms include extreme fatigue, breast tenderness and swelling, frequent urination, food aversions or cravings, heightened sense of smell, and mood changes.

These symptoms typically peak around weeks 8-10 and begin to improve as the first trimester ends. However, every pregnancy is different, and some women experience minimal symptoms while others find the first trimester quite difficult. It's important to communicate any concerns with healthcare providers, especially if symptoms are severe or interfering with daily life.

What Changes Occur During the Second Trimester?

The second trimester (weeks 13-27) is often called the "golden period" of pregnancy as morning sickness typically subsides and energy returns. The fetus grows significantly, reaching about 14 inches and 2 pounds by week 27, with clearly defined features and the ability to hear, see light, and respond to stimuli.

The second trimester marks a significant shift in the pregnancy experience for most women. As the placenta fully takes over hormone production, the dramatic hormonal fluctuations of early pregnancy stabilize, and many of the uncomfortable first trimester symptoms diminish or disappear entirely. Energy levels typically increase, and many women describe feeling their best during these middle months. This makes the second trimester an ideal time for travel, preparing the nursery, and enjoying the pregnancy before the physical demands of the third trimester begin.

Fetal development during the second trimester is characterized by rapid growth and the refinement of already-formed structures. During weeks 13-16, the fetus develops more defined features—eyes move to their final position on the face, ears reach their permanent location, and the neck becomes more distinct. The skeleton begins hardening from cartilage to bone, and the liver and pancreas start functioning. By week 16, the fetus is about 4-5 inches long and can make facial expressions.

Weeks 17-20 bring exciting developments for expectant parents. Around week 18-22, most women begin feeling fetal movement, often described as fluttering or bubbling sensations called "quickening." The anatomy scan, typically performed between weeks 18-22, provides a detailed look at fetal development and can reveal the baby's sex if parents wish to know. The fetus develops a protective coating called vernix caseosa and fine hair called lanugo that covers the body. Hearing develops, and the fetus begins responding to sounds.

During weeks 21-27, the fetus continues growing rapidly, reaching about 14 inches and 2 pounds by the end of the second trimester. The lungs are developing but not yet mature enough for breathing air. The brain is growing quickly, and the fetus establishes sleep-wake cycles. Eyes can open and close, and the fetus may respond to bright light shined on the abdomen. Fingerprints and footprints form, creating unique identification that will last a lifetime.

Physical Changes for the Mother

The second trimester brings visible changes as the uterus expands above the pelvis and a noticeable baby bump develops. Many women begin wearing maternity clothes around weeks 14-16. Other physical changes include skin changes such as the "pregnancy glow," darkening of the linea nigra (line down the abdomen), and possible development of stretch marks. Some women experience round ligament pain—sharp pains on the sides of the abdomen as ligaments stretch to accommodate the growing uterus.

Braxton Hicks contractions—irregular, usually painless tightening of the uterus—may begin during the second trimester, though they become more noticeable later. These "practice contractions" help prepare the uterus for labor but do not indicate that labor is beginning. Back pain may develop as the center of gravity shifts, and some women experience leg cramps, especially at night.

How Does the Third Trimester Prepare Baby for Birth?

The third trimester (weeks 28-40) focuses on final fetal maturation, including lung development crucial for breathing after birth, brain development, and accumulation of fat for temperature regulation. The baby typically settles into a head-down position in preparation for delivery.

The third trimester represents the final stretch of pregnancy, during which the fetus undergoes crucial final development in preparation for life outside the womb. While all major organs formed during the first trimester, the third trimester is essential for maturation—particularly of the lungs and brain—that enables survival after birth. A baby born at the start of the third trimester (28 weeks) has a survival rate of about 90% with modern neonatal care, but benefits from every additional week of development.

During weeks 28-32, the fetus grows rapidly, typically gaining about half a pound per week. The lungs begin producing surfactant, a substance essential for breathing that prevents the air sacs from collapsing. The brain develops rapidly, with neurons forming connections at an astonishing rate. The bones continue hardening, though the skull remains flexible to facilitate passage through the birth canal. The fetus practices breathing movements, swallowing amniotic fluid, and may have hiccups that the mother can feel.

Weeks 33-36 bring continued growth and development. Fat accumulates under the skin, filling out the fetus's previously wrinkled appearance and providing essential insulation for temperature regulation after birth. The digestive system is nearly mature, and the fetus is swallowing amniotic fluid and producing meconium (the first bowel movement). Most babies settle into a head-down position by week 36, preparing for delivery. The lanugo hair begins falling off, and the vernix coating thickens.

The final weeks (37-40) focus on finishing touches. The brain and lungs continue maturing right up until birth. The fetus gains about half a pound per week, with the average full-term baby weighing 6-9 pounds at birth. Antibodies pass from mother to baby, providing temporary immunity to infections. The baby drops lower into the pelvis (called "lightening" or "engagement"), which may ease breathing for the mother but increase pressure on the bladder.

Preparing for Labor and Delivery

As the due date approaches, the body begins preparing for labor through several signs and changes. The cervix begins to soften, thin (effacement), and may start to dilate. Braxton Hicks contractions may become more frequent and stronger. Some women experience a sudden burst of energy called "nesting instinct," feeling compelled to clean and organize the home. The mucus plug that sealed the cervix during pregnancy may be released, sometimes with a bloody show.

True labor is characterized by regular contractions that increase in frequency, duration, and intensity over time and do not subside with rest or position changes. Other signs that labor may be beginning include rupture of membranes ("water breaking") and persistent lower back pain. Understanding the difference between false labor (Braxton Hicks) and true labor helps expectant parents know when to contact their healthcare provider or head to the hospital.

Why Is Prenatal Care Important?

Regular prenatal care significantly reduces the risk of pregnancy complications and improves outcomes for both mother and baby. Prenatal visits monitor fetal growth, screen for potential problems, provide important vaccinations, and offer education and support throughout pregnancy.

Prenatal care is one of the most effective interventions in medicine for improving maternal and infant health outcomes. Regular monitoring throughout pregnancy allows healthcare providers to identify and address potential problems early, when intervention is most effective. Studies consistently show that women who receive adequate prenatal care have lower rates of preterm birth, low birth weight babies, and maternal mortality compared to those who receive limited or no care.

The typical prenatal care schedule includes monthly visits during the first and second trimesters (through week 28), visits every two weeks from weeks 28-36, and weekly visits from week 36 until delivery. Each visit includes measurements of blood pressure, weight, and fundal height (the distance from the pubic bone to the top of the uterus, which tracks fetal growth). The provider also checks fetal heart rate and, later in pregnancy, fetal position.

Prenatal screening and diagnostic tests are offered at specific points during pregnancy to assess the health of both mother and baby. First trimester screening (weeks 11-14) combines blood tests with an ultrasound to assess the risk of chromosomal abnormalities like Down syndrome. The anatomy scan (weeks 18-22) examines fetal development in detail. Glucose screening (weeks 24-28) tests for gestational diabetes. Group B streptococcus testing (weeks 35-37) determines if antibiotics are needed during labor to protect the baby.

Beyond physical monitoring, prenatal care provides essential education about nutrition, exercise, warning signs to watch for, labor preparation, breastfeeding, and newborn care. This knowledge empowers expectant parents to make informed decisions and recognize when to seek help. The relationship built with healthcare providers during prenatal visits also facilitates better communication and trust during labor and delivery.

Prenatal Vitamins and Nutrition

Proper nutrition during pregnancy supports optimal fetal development and maternal health. Prenatal vitamins serve as nutritional insurance, ensuring adequate intake of critical nutrients even when diet is less than perfect. The most important nutrient is folic acid, which reduces the risk of neural tube defects by up to 70% when taken before conception and during early pregnancy. The recommended dose is 400-800 mcg daily, though women with certain risk factors may need higher doses.

Other essential nutrients in prenatal vitamins include iron (to prevent anemia and support increased blood volume), calcium and vitamin D (for bone development), DHA omega-3 fatty acids (for brain and eye development), and iodine (for thyroid function and brain development). A balanced diet rich in fruits, vegetables, whole grains, lean proteins, and dairy products provides additional nutrients and fiber.

When Should You Seek Immediate Medical Care During Pregnancy?

Seek emergency care immediately for heavy vaginal bleeding, severe abdominal pain, sudden severe headache with vision changes, decreased fetal movement, sudden swelling of face or hands, fever over 101°F (38.3°C), or rupture of membranes before 37 weeks. These symptoms may indicate serious complications requiring prompt treatment.

While most pregnancies progress normally, knowing the warning signs of potential complications can be lifesaving for both mother and baby. Prompt recognition and treatment of serious conditions significantly improves outcomes. Every pregnant person should be familiar with the symptoms that require immediate medical attention and have a plan for reaching care quickly when needed.

Vaginal bleeding during pregnancy always warrants evaluation, though the urgency depends on the amount and accompanying symptoms. Light spotting in early pregnancy is common and often harmless, but heavy bleeding (soaking a pad in an hour or less) at any point requires immediate emergency care. Bleeding accompanied by severe pain, dizziness, or passing tissue is particularly concerning. In late pregnancy, bleeding may indicate placenta previa (placenta covering the cervix) or placental abruption (placenta separating from the uterine wall), both of which require urgent treatment.

Preeclampsia, a serious condition characterized by high blood pressure and protein in the urine, develops after 20 weeks in about 5-8% of pregnancies. Warning signs include severe headache that doesn't respond to acetaminophen, visual disturbances (blurred vision, seeing spots, sensitivity to light), sudden swelling of the face, hands, or feet, upper abdominal pain (especially on the right side), and decreased urine output. Preeclampsia can progress rapidly to life-threatening seizures (eclampsia) or HELLP syndrome without treatment.

Changes in fetal movement are one of the most important things for pregnant people to monitor, especially in the third trimester. While patterns of movement vary, a significant decrease in the baby's usual activity level may indicate fetal distress and should be evaluated promptly. Most providers recommend contacting them if you notice fewer than 10 movements in 2 hours during a time when the baby is usually active. Counting fetal movements ("kick counts") daily after 28 weeks can help detect problems early.

Is Exercise Safe During Pregnancy?

Yes, exercise is safe and beneficial during most pregnancies. The American College of Obstetricians and Gynecologists recommends 150 minutes of moderate-intensity aerobic activity per week. Regular exercise reduces the risk of gestational diabetes, preeclampsia, cesarean delivery, and excessive weight gain while improving mood and energy.

Physical activity during pregnancy offers numerous benefits for both mother and baby when performed safely. Research consistently shows that exercise during pregnancy reduces the risk of several complications, including gestational diabetes (by up to 30%), preeclampsia, excessive weight gain, and cesarean delivery. Exercise also helps manage common pregnancy discomforts like back pain, constipation, and swelling while improving mood, energy levels, and sleep quality.

For women who were physically active before pregnancy, continuing their exercise routine with appropriate modifications is generally safe and encouraged. For those who were sedentary, pregnancy is an excellent time to begin a moderate exercise program after consulting with a healthcare provider. The key is to start slowly, listen to your body, and avoid overexertion. Heart rate monitoring is no longer recommended as the primary guide; instead, the "talk test" (being able to carry on a conversation while exercising) indicates appropriate intensity.

Safe exercises during pregnancy include walking, swimming and water aerobics (excellent for reducing joint stress), stationary cycling, low-impact aerobics, prenatal yoga and Pilates, and strength training with appropriate modifications. Swimming and water exercise are particularly beneficial because the buoyancy reduces stress on joints while providing excellent cardiovascular conditioning and relief from swelling and overheating.

Certain activities should be avoided during pregnancy due to increased risk of injury or complications. Contact sports (soccer, basketball, hockey) pose risks from collisions and falls. Activities with high fall risk (skiing, horseback riding, cycling on roads) become more dangerous as the center of gravity shifts. Scuba diving is contraindicated due to decompression risks to the fetus. Hot yoga and exercising in hot, humid environments can lead to dangerous overheating. After the first trimester, exercises lying flat on the back for extended periods should be avoided as the weight of the uterus can compress major blood vessels.

Exercise Precautions and When to Stop

Stop exercising and contact your healthcare provider if you experience vaginal bleeding, fluid leaking from the vagina, regular painful contractions, dizziness or feeling faint, shortness of breath before starting exercise, chest pain, headache, calf pain or swelling, or muscle weakness affecting balance. Women with certain conditions, including some heart and lung diseases, cervical insufficiency, placenta previa, preeclampsia, severe anemia, or being pregnant with multiples at risk for preterm labor, may need to limit or avoid exercise.

What Should You Eat and Avoid During Pregnancy?

A balanced pregnancy diet includes fruits, vegetables, whole grains, lean proteins, and dairy products, with increased needs for protein, iron, calcium, and certain vitamins. Avoid raw or undercooked meats and eggs, unpasteurized dairy, high-mercury fish, alcohol, and limit caffeine to 200mg daily.

Proper nutrition during pregnancy supports the remarkable work of growing a new human being while maintaining the mother's health. Caloric needs increase modestly—about 340 extra calories per day in the second trimester and 450 in the third (equivalent to a healthy snack or small meal). The saying "eating for two" is misleading; what matters more than quantity is quality—ensuring adequate intake of essential nutrients for fetal development.

Protein needs increase during pregnancy to support the growth of fetal tissues, the placenta, and increased maternal blood volume. Good protein sources include lean meats, poultry, fish (following safe guidelines), eggs, beans and lentils, tofu, nuts, and dairy products. Aim for about 75-100 grams of protein daily during the second and third trimesters. Iron requirements also increase significantly, from 18mg to 27mg daily, to support expanded blood volume and prevent anemia. Iron-rich foods include red meat, fortified cereals, beans, and dark leafy greens.

Calcium and vitamin D work together to build the baby's bones and teeth while protecting maternal bone density. If the diet doesn't provide enough calcium, the body will draw from the mother's bones to meet fetal needs. Dairy products, fortified plant milks, calcium-set tofu, and dark leafy greens are good sources. Most prenatal vitamins contain vitamin D, and some sun exposure also helps. DHA omega-3 fatty acids, found in fatty fish, walnuts, and fortified foods, support fetal brain and eye development.

Certain foods pose risks during pregnancy due to potential contamination with harmful bacteria or parasites, or because they contain substances that can harm fetal development. Raw or undercooked meat, poultry, eggs, and seafood may contain Salmonella, E. coli, Listeria, or Toxoplasma parasites. Unpasteurized dairy products and soft cheeses made from unpasteurized milk can harbor Listeria. High-mercury fish (shark, swordfish, king mackerel, tilefish, bigeye tuna) can affect fetal nervous system development. Alcohol has no known safe amount during pregnancy and can cause fetal alcohol spectrum disorders.

• Eat plenty of: Fruits, vegetables, whole grains, lean proteins, dairy
• Increase intake of: Iron, calcium, folic acid, protein, DHA
• Limit: Caffeine to 200mg/day (about one 12oz coffee)
• Avoid: Alcohol, raw meats/eggs, unpasteurized dairy, high-mercury fish

How Does Pregnancy Affect Mental Health?

Pregnancy brings significant emotional changes due to hormonal shifts, life adjustments, and physical symptoms. While mood swings are normal, about 10-20% of pregnant women experience depression or anxiety requiring treatment. Support, self-care, and professional help when needed are essential for maternal and fetal well-being.

The emotional landscape of pregnancy encompasses a wide range of feelings, from joy and excitement to anxiety, fear, and occasional sadness. Hormonal changes, physical discomforts, sleep disruption, and the psychological weight of impending parenthood all contribute to emotional fluctuations. Understanding that these feelings are normal and common can provide reassurance, but it's equally important to recognize when emotions cross the line from normal adjustment to conditions requiring professional support.

Prenatal depression and anxiety affect more women than commonly recognized—studies suggest 10-20% of pregnant women experience clinically significant depression, and anxiety disorders are even more common. Risk factors include a personal or family history of mental health conditions, previous pregnancy loss, unplanned pregnancy, relationship difficulties, limited social support, stressful life events, and complications during pregnancy. These conditions can affect both maternal health and fetal development if left untreated.

Symptoms of prenatal depression may include persistent sadness or hopelessness, loss of interest in activities once enjoyed, changes in appetite or sleep beyond normal pregnancy changes, difficulty concentrating or making decisions, feelings of worthlessness or excessive guilt, thoughts of death or suicide, and withdrawal from relationships. Anxiety may manifest as persistent excessive worry, physical symptoms like racing heart or shortness of breath, panic attacks, obsessive thoughts, or avoidance behaviors.

Treatment for perinatal mental health conditions is effective and safe during pregnancy. Options include psychotherapy (particularly cognitive behavioral therapy), support groups, lifestyle modifications, and when indicated, medication. Many antidepressants have been studied extensively during pregnancy and are considered safe when the benefits outweigh any potential risks. Untreated depression and anxiety may pose greater risks to the pregnancy than appropriate treatment. If you're experiencing symptoms, speak with your healthcare provider promptly.

Self-Care Strategies for Emotional Well-Being

Several strategies support emotional health during pregnancy. Regular physical activity releases endorphins and reduces stress. Adequate sleep (though often challenging) helps regulate mood. Building and maintaining social connections provides crucial support. Prenatal classes and support groups connect you with others going through similar experiences. Practicing relaxation techniques like deep breathing, meditation, or prenatal yoga can reduce anxiety. Setting realistic expectations and accepting help from others eases the transition to parenthood.