Pregnancy Week by Week — Complete Trimester Guide
Blog › Health · 9 min read · Published 2026-03-16
Track your pregnancy journey week by week. Understand each trimester, developmental milestones, and what to expect at every stage.
Understanding Pregnancy Timing
Pregnancy is measured from the first day of your last menstrual period (LMP), which means you're considered "2 weeks pregnant" at the time of conception. A full-term pregnancy lasts 40 weeks (280 days), divided into three trimesters. This dating method, called Naegele's Rule, was established in the early 1800s and remains the standard used by obstetricians worldwide.
Only about 5% of babies are born on their exact due date. Most births occur between weeks 37 and 42, all considered "term." Use our Pregnancy Week Calculator to track exactly where you are in your journey.
First Trimester (Weeks 1-12)
The first trimester is a period of rapid development. By week 4, the embryo has implanted in the uterus. By week 6, the heartbeat is detectable on ultrasound. All major organs and body systems begin forming during this critical period.
Key Milestones
- Week 4: Implantation complete. Pregnancy hormone (hCG) detectable on tests.
- Week 6: Heart begins beating. Neural tube (future brain and spine) forms.
- Week 8: All major organs are forming. Fingers and toes begin developing.
- Week 10: Embryo is now called a fetus. All vital organs are present.
- Week 12: End of first trimester. Risk of miscarriage drops significantly.
Common Symptoms
Morning sickness (which can occur any time of day), fatigue, breast tenderness, frequent urination, and food aversions are all common. These symptoms are caused by rising hormone levels and usually improve in the second trimester.
Second Trimester (Weeks 13-26)
Often called the "golden period," the second trimester typically brings relief from first-trimester symptoms. Energy returns, and many women feel their best during this stage.
Key Milestones
- Week 14: Baby's sex organs are developing. Morning sickness often subsides.
- Week 16: Baby's sex may be visible on ultrasound. Mother may feel "quickening" (first movements).
- Week 18: Baby can hear sounds from outside the womb.
- Week 20: Anatomy scan ultrasound — the halfway point. Baby is about 6.5 inches long.
- Week 24: Viability milestone. With intensive medical care, babies born at this stage have a chance of survival.
Important Screenings
The anatomy scan at weeks 18-20 is one of the most important prenatal appointments. It checks the baby's organs, growth, placenta position, and amniotic fluid levels. Genetic screening tests (quad screen, NIPT) are typically offered between weeks 15-20.
Third Trimester (Weeks 27-40)
The final stretch focuses on rapid fetal growth. The baby gains most of its birth weight during this period, growing from about 2 pounds at week 27 to an average of 7-8 pounds at birth.
Key Milestones
- Week 28: Eyes can open and close. Baby responds to light and sound.
- Week 32: Baby practices breathing movements. Bones are hardening.
- Week 34: Lungs are nearly mature. Baby is gaining about half a pound per week.
- Week 36: Baby is considered "early term." Most babies are head-down by now.
- Week 37: Full term begins. Baby is considered mature enough for birth.
- Week 40: Estimated due date. Labor can begin anytime between weeks 37-42.
Common Symptoms
Back pain, Braxton Hicks contractions, difficulty sleeping, swelling, shortness of breath, and frequent urination return as the baby grows. These are normal but should be discussed with your healthcare provider.
Prenatal Care Schedule
Standard prenatal visit frequency:
- Weeks 4-28: Every 4 weeks (monthly)
- Weeks 28-36: Every 2 weeks (biweekly)
- Weeks 36-40: Every week
Each visit typically includes blood pressure, weight check, fundal height measurement, and fetal heart rate monitoring. Additional tests are scheduled at specific weeks throughout pregnancy.
When to Call Your Provider
Contact your healthcare provider immediately if you experience: vaginal bleeding, severe abdominal pain, sudden severe headache, vision changes, decreased fetal movement, fluid leaking, or regular contractions before 37 weeks.
FAQ
- How is my due date calculated? Your due date is estimated at 280 days (40 weeks) from the first day of your last menstrual period. First-trimester ultrasound can adjust this estimate if your cycle is irregular.
- What if I don't know my LMP date? An early ultrasound (ideally before 12 weeks) can accurately date your pregnancy by measuring the fetus's crown-rump length.
- Is it normal to go past my due date? Yes. Up to 42 weeks is considered within the normal range, though most providers recommend induction by 41-42 weeks.
- How much weight should I gain? The recommended weight gain depends on your pre-pregnancy BMI: Normal BMI (18.5-24.9): 25-35 lbs. Overweight (25-29.9): 15-25 lbs. Obese (30+): 11-20 lbs. Use our Pregnancy Weight Gain Calculator for personalized targets.
Disclaimer: This guide is for educational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider for personalized guidance during pregnancy.