Pregnancy Week by Week: Your Complete Development Guide

How Is Pregnancy Calculated and Dated?

Pregnancy is calculated from the first day of your last menstrual period (LMP), not from conception. This means when you are "4 weeks pregnant," the embryo is actually only about 2 weeks old. A full-term pregnancy lasts 40 weeks (280 days) from your LMP, divided into three trimesters.

Understanding how pregnancy is dated can be confusing for many expectant parents. Healthcare providers use the first day of your last menstrual period as the starting point because it's a date most women can remember accurately, while the exact moment of conception is usually unknown. This dating method, known as gestational age, is used worldwide and forms the basis of all pregnancy care guidelines.

Conception typically occurs about two weeks after your last period, during ovulation. This means that during "week 1" and "week 2" of pregnancy, you're not actually pregnant yet - your body is preparing for ovulation and the potential fertilization of an egg. The embryo doesn't implant into the uterine wall until about a week after conception, around week 3-4 of gestational age.

Your estimated due date is calculated by adding 280 days (40 weeks) to the first day of your last period, or by using Naegele's rule: subtract three months from your LMP and add seven days. For example, if your last period started on January 1st, your due date would be approximately October 8th. However, only about 5% of babies are born on their exact due date - most arrive within two weeks before or after.

Confirming Your Due Date with Ultrasound

While LMP-based dating provides an initial estimate, ultrasound scans offer more accurate dating, especially in the first trimester. A dating scan performed between weeks 8-14 can confirm gestational age by measuring the embryo's crown-rump length. This measurement is particularly valuable if you have irregular periods or are unsure of your LMP date. If there's more than a week's difference between ultrasound dating and LMP dating, your healthcare provider may adjust your due date.

What Happens During the First Trimester (Weeks 1-12)?

The first trimester is the most critical period for fetal development. All major organs, the brain, spinal cord, heart, and limbs begin forming. By week 12, your baby has all essential body structures in place, though they're still tiny - about the size of a lime. Many women experience morning sickness, fatigue, and breast tenderness during this time.

The first trimester encompasses weeks 1 through 12 and is arguably the most transformative period of pregnancy. During these crucial early weeks, a single fertilized egg develops into an embryo with recognizable human features. The rapid cell division and organ formation during this time make the first trimester a period when the developing baby is most vulnerable to environmental factors, which is why prenatal vitamins and avoiding harmful substances are particularly important.

From the mother's perspective, the first trimester often brings significant physical and emotional changes. Rising hormone levels, particularly human chorionic gonadotropin (hCG) and progesterone, trigger many common early pregnancy symptoms. While some women sail through these early weeks with minimal discomfort, others experience challenging symptoms that can affect daily life. The good news is that most first-trimester symptoms improve significantly once you enter the second trimester.

Your first prenatal appointment typically occurs around week 8-10, during which your healthcare provider will confirm the pregnancy, estimate your due date, order blood tests, and discuss your health history. This visit establishes the foundation for your prenatal care journey and is an opportunity to ask questions about what to expect in the coming months.

Weeks 1-4: Conception and Implantation

During weeks 1-2, your body prepares for ovulation. Conception occurs around week 2-3 when sperm fertilizes the egg. The fertilized egg, now called a zygote, begins dividing rapidly as it travels down the fallopian tube toward the uterus. By week 4, the blastocyst implants into the uterine lining, and the placenta begins forming. Many women don't yet know they're pregnant during this time, though some may experience light spotting called implantation bleeding.

Weeks 5-8: Embryonic Development

Week 5 marks when many women miss their period and may first suspect pregnancy. The embryo's heart begins to beat around week 5-6, and by week 8, all major organs have begun forming. The neural tube, which becomes the brain and spinal cord, closes during week 6. Tiny arm and leg buds appear, and facial features like eyes and ears start developing. By the end of week 8, the embryo is about the size of a raspberry and is officially called a fetus.

Weeks 9-12: Fetal Development Advances

During weeks 9-12, your baby transitions from embryo to fetus and begins looking more recognizably human. Fingers and toes separate, bones begin hardening, and external genitalia start differentiating (though it's usually too early to determine sex by ultrasound). By week 12, your baby can make reflexive movements, though you won't feel them yet. The risk of miscarriage decreases significantly after week 12, which is why many parents choose to announce their pregnancy at this milestone.

What Changes Occur During the Second Trimester (Weeks 13-27)?

The second trimester, often called the "honeymoon phase," brings relief from first-trimester symptoms for many women. Your baby grows rapidly, develops facial expressions, and begins moving noticeably. You'll likely feel those first magical kicks between weeks 16-25. An anatomy scan around week 20 can reveal your baby's sex if you wish to know.

The second trimester is frequently described as the most comfortable period of pregnancy. For many women, the nausea and fatigue of the first trimester fade, energy levels increase, and the pregnancy begins to feel more "real" as the baby bump becomes visible. This middle phase of pregnancy, spanning weeks 13 through 27, is characterized by rapid fetal growth and exciting developmental milestones.

During this trimester, your baby's movements become strong enough to feel. First-time mothers typically notice these sensations, called "quickening," between weeks 18-25, while women who have been pregnant before may recognize the fluttering as early as week 14-16. What starts as gentle butterfly-like sensations gradually becomes unmistakable kicks and rolls as your baby grows stronger. By the end of the second trimester, partners may be able to feel movements by placing their hand on your belly.

The mid-pregnancy anatomy scan, typically performed around weeks 18-22, is a highlight of the second trimester. This detailed ultrasound examines your baby's major organs, measures growth, checks the placenta location, and can reveal the baby's sex if you want to know. This scan also screens for certain structural abnormalities, providing reassurance about your baby's development.

Weeks 13-16: Entering the Second Trimester

As you enter the second trimester, your baby's facial features become more defined, and the body catches up with the proportionally large head. Bones continue hardening, and your baby practices breathing by inhaling amniotic fluid. By week 14, the sex may be visible on ultrasound, though confirmation usually waits until the anatomy scan. Your uterus is now rising above your pubic bone, and you may start "showing" around week 14-16.

Weeks 17-20: Feeling Baby's Movements

Many women feel their baby move for the first time during these weeks - a milestone that makes pregnancy feel wonderfully real. Your baby develops a layer of vernix caseosa (a waxy coating) protecting their skin from the amniotic fluid. By week 18, your baby can hear sounds from outside the womb, so talking and singing to your bump isn't just sentimental - your baby is listening. The anatomy scan around week 20 provides detailed images of your growing baby.

Weeks 21-27: Viability Milestone

Week 24 marks an important milestone: viability. While premature birth is still serious, medical advances mean babies born at 24 weeks have a chance of survival with intensive care. Your baby's brain develops rapidly during these weeks, and they practice sucking and swallowing reflexes. Eyes can open by week 26, and your baby responds to light and sound. You'll notice definite patterns in your baby's movements, including active and quiet periods.

What Should You Expect During the Third Trimester (Weeks 28-40)?

The third trimester is the final stretch before meeting your baby. Your baby gains weight rapidly, organs mature for life outside the womb, and the head typically moves into the birth position. You may experience Braxton Hicks contractions, increased discomfort, and frequent urination as your baby grows and prepares for birth.

The third trimester, spanning weeks 28 through 40, is a time of anticipation and preparation. Your baby is growing rapidly, putting on about half a pound per week, and their organs are maturing in preparation for life outside the womb. While this trimester can bring increased physical discomfort as your baby takes up more room, it's also an exciting time as you prepare for labor, delivery, and parenthood.

Your prenatal appointments become more frequent during the third trimester, typically every two weeks from week 28-36 and weekly thereafter. These visits monitor your baby's position, your blood pressure (screening for preeclampsia), and signs that labor may be approaching. Group B streptococcus testing around week 36-37 determines whether you'll need antibiotics during labor. Your healthcare provider will also discuss your birth preferences and signs of labor to watch for.

Counting your baby's movements becomes especially important during the third trimester. Your healthcare provider will likely recommend daily kick counting after week 28, as a decrease in movement can indicate fetal distress. While your baby's movements may feel different as space becomes limited (more rolls and stretches than kicks), you should still feel regular activity. Contact your healthcare provider immediately if you notice a significant decrease in movement.

Weeks 28-32: Baby Grows Rapidly

Your baby's brain is developing complex connections, and they can now dream during REM sleep. Bones are fully developed but still soft enough for the birth process. Your baby practices breathing movements, and their eyes can focus. Space becomes tighter, so while movements may feel different, they should remain regular. You might notice your baby responding to familiar voices, music, and even spicy food!

Weeks 33-36: Preparing for Birth

Most babies move into a head-down position during these weeks in preparation for birth. The lungs continue maturing, producing surfactant essential for breathing air. Your baby gains about 230 grams (half a pound) per week, and their immune system strengthens. You may experience "lightening" as the baby drops lower into your pelvis, making breathing easier but increasing pressure on your bladder.

Weeks 37-40: Full Term and Ready

Your baby is now considered "early term" at 37-38 weeks and "full term" at 39-40 weeks. Lungs are typically fully mature, and your baby continues adding fat stores for temperature regulation after birth. You may experience increasing Braxton Hicks contractions, lose your mucus plug, or notice other signs that labor is approaching. Most women go into labor between weeks 38-42, with first-time mothers often delivering slightly later.

How Does Fetal Development Progress Week by Week?

Fetal development follows a remarkable progression from a single cell to a fully formed baby. The embryonic period (weeks 3-8) establishes all major organ systems, while the fetal period (weeks 9-40) focuses on growth and maturation. Key milestones include heartbeat (week 5-6), movement (week 8), hearing (week 18), and lung maturity (week 36).

The journey from fertilization to birth involves intricate developmental processes occurring in a precisely timed sequence. Understanding these milestones can help you appreciate the remarkable transformation happening within your body and provides context for the tests and monitoring your healthcare provider recommends at different stages.

Development occurs in two main phases: the embryonic period (weeks 3-8) and the fetal period (weeks 9 onwards). During the embryonic period, the basic body plan is established, and all major organ systems begin forming. This is when the developing baby is most susceptible to teratogens - substances that can cause birth defects. The fetal period focuses on growth and maturation of already-formed structures, with the baby becoming increasingly capable of surviving outside the womb as the weeks progress.

While development follows a general timeline, there's natural variation between pregnancies. Ultrasound measurements that fall within a range of normal don't indicate problems. Your healthcare provider monitors growth trends over time rather than focusing on single measurements, ensuring your baby is developing appropriately for their gestational age.

What Are Common Pregnancy Symptoms by Week?

Pregnancy symptoms vary by trimester. First trimester brings nausea, fatigue, and breast tenderness. Second trimester often provides relief with increased energy. Third trimester symptoms include backache, heartburn, swelling, and Braxton Hicks contractions. Not all women experience every symptom, and severity varies significantly.

Pregnancy symptoms result from the dramatic hormonal changes occurring in your body. Understanding which symptoms are normal - and when to contact your healthcare provider - helps you navigate these nine months with confidence. While symptoms can be uncomfortable, they're usually signs that your pregnancy is progressing normally.

It's important to remember that every pregnancy is unique. Some women experience severe morning sickness while others feel perfectly fine. Some develop every textbook symptom while others sail through with minimal discomfort. The presence or absence of symptoms doesn't indicate the health of your pregnancy, so try not to compare your experience to others'.

First Trimester Symptoms

Morning sickness affects up to 80% of pregnant women and, despite its name, can occur at any time of day. It typically begins around week 6, peaks around weeks 9-10, and usually improves by week 12-14. Severe nausea and vomiting (hyperemesis gravidarum) requires medical attention. Fatigue is overwhelming for many women in early pregnancy as your body works overtime to support the growing embryo. Breast tenderness and enlargement result from hormonal changes preparing for breastfeeding. Frequent urination occurs as your uterus expands and presses on your bladder.

Second Trimester Symptoms

Many women find the second trimester more comfortable as first-trimester symptoms fade. However, new symptoms emerge as your belly grows. Round ligament pain causes sharp pain in your lower abdomen or groin as the ligaments supporting your uterus stretch. Backache develops as your center of gravity shifts forward. Skin changes include the linea nigra (dark line down your belly), stretch marks, and the "pregnancy glow" from increased blood flow. Braxton Hicks contractions may begin - these "practice contractions" are irregular and usually painless.

Third Trimester Symptoms

The final trimester brings symptoms related to your baby's size and position. Shortness of breath occurs as your uterus pushes against your diaphragm, though this may improve after "lightening" when your baby drops lower. Heartburn worsens as digestive organs are compressed. Swelling (edema) in feet and ankles is common, though sudden or severe swelling warrants medical attention. Difficulty sleeping results from finding a comfortable position, frequent urination, and the baby's movements. Pelvic pressure increases as your baby's head descends into the birth canal.

What Prenatal Care Is Needed During Pregnancy?

Regular prenatal care is essential for a healthy pregnancy. Appointments typically occur monthly until week 28, biweekly until week 36, then weekly until birth. Care includes blood tests, urine tests, blood pressure monitoring, ultrasound scans, screening tests for chromosomal conditions, and continuous monitoring of fetal growth and maternal health.

Prenatal care encompasses all the medical care you receive during pregnancy. Regular appointments with your healthcare provider - whether an obstetrician, midwife, or family physician - allow for monitoring of both your health and your baby's development. These visits also provide opportunities to discuss concerns, receive guidance on nutrition and lifestyle, and prepare for labor and delivery.

The schedule and content of prenatal visits follow evidence-based guidelines designed to detect potential complications early when intervention is most effective. While low-risk pregnancies may require fewer visits, high-risk pregnancies may need more frequent monitoring. Your healthcare provider will tailor your care plan to your individual needs and risk factors.

First Trimester Prenatal Care

Your first prenatal visit, ideally scheduled between weeks 8-10, is typically the longest. Your healthcare provider will confirm pregnancy, estimate your due date, take a complete medical history, and order initial blood tests. These tests check blood type and Rh factor, immunity to rubella, hepatitis B and C status, HIV status, and screen for anemia and other conditions. A dating ultrasound may be performed if your LMP is uncertain. First-trimester screening for chromosomal abnormalities (weeks 11-14) combines blood tests with a nuchal translucency ultrasound.

Second Trimester Prenatal Care

Monthly appointments continue through the second trimester. At each visit, your healthcare provider checks your blood pressure, weight, urine (for protein and glucose), and measures your fundal height (distance from pubic bone to top of uterus). The anatomy scan around weeks 18-22 provides detailed evaluation of fetal development. Additional blood tests may include screening for gestational diabetes (glucose tolerance test) around weeks 24-28 and anemia screening.

Third Trimester Prenatal Care

Visits increase to biweekly from week 28-36, then weekly until delivery. Your healthcare provider monitors your baby's position, estimates size, and assesses signs of labor. Group B streptococcus (GBS) screening around weeks 36-37 determines whether you need antibiotics during labor. Non-stress tests or biophysical profiles may be ordered if there are concerns about fetal well-being. These final visits also focus on preparing for labor, discussing birth preferences, and recognizing when to go to the hospital.

When Should You Seek Emergency Care During Pregnancy?

Seek immediate medical care for heavy vaginal bleeding, severe abdominal pain, significant decrease in fetal movement, severe headache with vision changes (possible preeclampsia), water breaking before 37 weeks, signs of infection (fever, chills), or difficulty breathing. Trust your instincts - if something feels wrong, contact your healthcare provider.

While most pregnancies progress normally, knowing the warning signs of complications allows you to seek timely care. Some situations require immediate medical attention, while others warrant a same-day call to your healthcare provider. When in doubt, it's always better to be checked and reassured than to delay care for a potentially serious condition.

Keep your healthcare provider's contact information readily available, and know where your nearest emergency department is. Many hospitals have dedicated labor and delivery triage units where pregnant women can be assessed quickly. After hours, your healthcare provider's office should have an on-call service for urgent concerns.

Preeclampsia Warning Signs

Preeclampsia is a serious pregnancy complication characterized by high blood pressure and protein in urine, typically developing after week 20. Warning signs include severe headaches that don't respond to medication, vision changes (blurred vision, seeing spots, or light sensitivity), upper abdominal pain (especially right side), sudden swelling of face and hands, and significant decrease in urination. Preeclampsia can progress to eclampsia (seizures) and requires immediate medical attention. Your blood pressure and urine are checked at every prenatal visit specifically to screen for this condition.

Frequently Asked Questions About Pregnancy Week by Week