Pregnancy Week by Week
Slide through weeks 1–40 to see your baby’s size, development milestones, and what to expect in your body — medically reviewed.
Baby’s Development
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Pregnancy Trimesters at a Glance
First Trimester
Major organs and body systems form. Common symptoms include fatigue, nausea, breast tenderness, and frequent urination as hCG and progesterone rise.
Second Trimester
Often called the “golden trimester.” Energy returns, the bump becomes visible, and you may feel the first flutters of fetal movement (quickening).
Third Trimester
Rapid weight gain, Braxton Hicks contractions, and final lung/brain maturation as your body prepares for labor and delivery.
How Pregnancy Weeks Are Counted
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the date of conception — which typically happens about two weeks later. This means that in week 4, your baby is usually only about two weeks past fertilization. This standardized method, recommended by the American College of Obstetricians and Gynecologists (ACOG), allows healthcare providers to track fetal growth consistently even when the exact conception date is unknown. If you conceived via IVF or know your ovulation date precisely, your provider may fine-tune your dating using ultrasound crown-rump length (CRL) measurements taken in the first trimester, which are considered the most accurate dating method.
What to Expect Each Trimester
Every pregnancy is unique, but most follow a broadly predictable arc of physical and emotional changes. In the first trimester, your body is doing enormous work behind the scenes — implanting the embryo, forming the placenta, and laying down the foundation for every organ system — even though outward signs may be subtle. By the second trimester, many of the early discomforts ease as the placenta takes over hormone production, energy levels often rebound, and most people choose to share their pregnancy news and schedule their anatomy scan (typically between 18–22 weeks). The third trimester brings the practical work of preparing for birth: monitoring fetal position, attending more frequent prenatal visits, and watching for signs of labor as your due date approaches.
Key Prenatal Milestones by Week
- Week 6–8: First prenatal visit and dating ultrasound; fetal heartbeat often detectable.
- Week 10–13: First-trimester screening / NIPT blood test option for chromosomal conditions.
- Week 18–22: Detailed anatomy ultrasound; anatomy and sex can typically be confirmed.
- Week 24–28: Glucose screening test for gestational diabetes; Tdap and RhoGAM (if Rh-negative) often given.
- Week 28–36: Visits increase to every 2 weeks; Group B strep test around week 36–37.
- Week 36–40: Weekly visits; provider checks baby’s position and cervix readiness for labor.
Medically Reviewed by Dr. Ruqaiya Khan, MD, FACOG
Board-certified OB-GYN. This content was reviewed for medical accuracy in line with ACOG and WHO clinical guidance. View our medical review process →
Related Reading
- First Trimester Guide — weeks 1 to 13
- Second Trimester Guide — weeks 14 to 27
- Third Trimester Guide — weeks 28 to birth
- 10 Weeks Pregnant — baby development and symptoms
Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.
More Pregnancy Stage Guides
- Pregnancy Week by Week — a full timeline of pregnancy
- First Trimester Guide — weeks 1 to 13
- Second Trimester Guide — weeks 14 to 27
- Third Trimester Guide — weeks 28 to birth
- 10 Weeks Pregnant — baby development and symptoms
- 13 Weeks Pregnant — end of the first trimester
- 21 Weeks Pregnant — mid-pregnancy milestones
- 32 Weeks Pregnant — preparing for the final stretch
- Pregnancy Symptoms by Week — what to expect as weeks pass
- Morning Sickness Guide — causes and relief options
- Pregnancy Swelling Guide — what is normal and when to call
- Pregnancy Back Pain Guide — causes and ways to ease it
- Labor Guide — signs, stages and preparation
- Breastfeeding Preparation — getting ready before the baby arrives
- Newborn Percentile Calculator — track growth after birth
For tools, try the pregnancy calculators; for dating questions, see the due date guide.
Frequently Asked Questions
Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not the date of conception. This adds roughly two weeks to your “true” fetal age, which is why a 40-week pregnancy is about 38 weeks from conception.
A full-term pregnancy is defined as 39 weeks 0 days through 40 weeks 6 days. Babies born between 37–38 weeks 6 days are considered “early term,” and those after 41 weeks are “late term.”
Most first-time mothers feel initial flutters (quickening) between weeks 18–22, while those who’ve been pregnant before may notice movement slightly earlier, around weeks 16–18.
Gestational age is measured from the LMP and is what your due date is based on. Fetal (conceptional) age is measured from the actual date of fertilization and is roughly two weeks behind gestational age.
They’re a helpful visual approximation based on average fetal length and weight percentiles published by WHO fetal growth references — actual size varies normally between individual pregnancies.
Sources: American College of Obstetricians and Gynecologists (ACOG), World Health Organization (WHO) Fetal Growth Charts, Centers for Disease Control and Prevention (CDC), American Academy of Pediatrics (AAP).
Why Follow Pregnancy Week by Week
Following your pregnancy week by week helps you understand what’s happening with your baby’s growth and your own body at every stage, from early development milestones to third-trimester changes. Pair this guide with our Due Date Calculator to know exactly which week you’re in. For authoritative guidance on prenatal care throughout each trimester, visit the American College of Obstetricians and Gynecologists.