Pregnancy, week by week.
A clear guide to what's changing, what's coming, and what deserves your attention.
How pregnancy weeks are counted
Pregnancy is measured from the first day of your last menstrual period (LMP), not from conception — so you are counted as "pregnant" for about two weeks before the egg is fertilized. A full-term pregnancy is about 40 weeks, and term is 37 to 42 weeks.
- Length About 40 weeks from your last period (280 days).
- Trimesters First 1–13, second 14–27, third 28 to birth.
- Gestational age Counts from the last period; fetal age is about two weeks less.
Find your week
Dating begins
Pregnancy is counted from the first day of your last period, so this week precedes conception.
You are not yet pregnant in the biological sense; the count is a shared dating convention.
Note the first day of your last period — your provider will date the pregnancy from it.
First trimester — weeks 1–13
Second trimester — weeks 14–27
Third trimester — weeks 28–42
Week-specific articles are being written and clinically reviewed. Until each is published, this timeline is the canonical summary for every week.
First trimester: weeks 1–13
Early development, the first symptoms, and the appointments and screening decisions that arrive sooner than most expect.
The foundation
By the end of the first trimester, the major organs have formed and the risk of miscarriage has fallen markedly. Most of the work now is confirming the pregnancy, starting prenatal care, and deciding which genetic screening, if any, is right for you.
- Start a prenatal vitamin with folic acid
- Book your first prenatal appointment (usually weeks 8–10)
- Discuss first-trimester and NT screening (weeks 11–13)
- Note any food-safety and medication questions for your provider
Second trimester: weeks 14–27
Growth and movement, the anatomy ultrasound, and the window where the big decisions are best made while there is time to plan.
The decisions begin
The second trimester is often the steadiest stretch. The detailed anatomy ultrasound (weeks 18–22) reviews growth and development, many people feel movement for the first time, and gestational-diabetes screening is offered toward the end (weeks 24–28).
- Attend the anatomy ultrasound (weeks 18–22)
- Note when you first feel regular movement
- Plan glucose screening (weeks 24–28)
- Choose your birth setting and provider while there is time
Third trimester: week 28 to birth
Final development, closer monitoring, birth preparation, and learning the signs of labor.
Preparing for the day
Appointments become more frequent, the baby gains weight and usually settles head-down, and group B strep screening is done near the end (weeks 36–37). Now is the time to finish a birth plan and learn when to call your team.
- Track daily fetal movement and report changes
- Write and discuss your birth plan
- Group B strep screening (weeks 36–37)
- Learn the signs of labor and when to call
What happens at prenatal appointments?
A typical schedule of care milestones. Your own provider sets the exact timing, which varies with your health and pregnancy.
| Milestone | Typical timing | Purpose | Notes |
|---|---|---|---|
| First prenatal visit | Weeks 8–10 | Confirm and date the pregnancy, health history, initial bloodwork | Timing varies; call early to schedule |
| First-trimester & NT screening | Weeks 11–13 | Optional screening for certain chromosomal conditions | Combined blood test and ultrasound |
| Cell-free DNA (NIPT) | From ~week 10 | Optional blood screening | Offered based on your preferences and risk |
| Anatomy ultrasound | Weeks 18–22 | Detailed check of growth and anatomy | Sex can often be identified if you wish |
| Glucose screening | Weeks 24–28 | Screen for gestational diabetes | Earlier if you have risk factors |
| Anti-D (RhoGAM) | ~Week 28 | For people who are Rh-negative | Prevents Rh sensitization |
| Group B strep (GBS) | Weeks 36–37 | Screen to guide antibiotics in labor if positive | A simple swab |
| Weekly visits | Week ~36 to birth | Monitor position, blood pressure, and well-being | More frequent as the due date nears |
Screening is a personal choice, and schedules differ between providers and countries. None of these tests is mandatory; your care team can explain what each one does and does not tell you.
Pregnancy symptoms that need medical attention
Most pregnancy discomforts are normal. A small number of symptoms mean contact your provider or seek urgent care the same day.
Call your provider or seek urgent care if you have:
- Vaginal bleeding, or leaking fluid from the vagina
- Severe or persistent headache, or vision changes
- Sudden swelling of the face or hands, or pain high in the abdomen
- A clear decrease in your baby's movements in the third trimester
- Regular, painful contractions before 37 weeks
- Fever, painful urination, or a feeling that something is seriously wrong
This is not a complete list. When in doubt, call — your provider would rather hear from you.
Questions worth asking your provider
- Which screening tests do you recommend for me, and why?
- What are my options for where and how to give birth?
- How should I track my baby's movements, and when should I call?
- What symptoms should prompt me to contact you right away?
Frequently asked questions
How many weeks is pregnancy?
About 40 weeks, counted from the first day of your last menstrual period. Term is 37 to 42 weeks; birth before 37 weeks is considered preterm.
When does each trimester begin?
Under the common convention, the first trimester is weeks 1–13, the second is weeks 14–27, and the third is week 28 to birth. Some resources draw the lines slightly differently.
How is a due date calculated?
The standard estimate adds 280 days (40 weeks) to the first day of your last period. An early ultrasound or your clinician's dating may adjust it, and only about one in twenty babies arrives exactly on the estimated date.
When can fetal movement usually be felt?
Many people first feel movement between about 16 and 25 weeks — often earlier in a later pregnancy and later in a first one. The absence of movement early on does not mean anything is wrong.
When are common prenatal tests offered?
First-trimester and NT screening at weeks 11–13, the anatomy ultrasound at 18–22, gestational-diabetes screening at 24–28, and group B strep at 36–37. Your provider sets the exact schedule.
Sources
- American College of Obstetricians and Gynecologists (ACOG). Prenatal care and routine tests during pregnancy. acog.org
- Centers for Disease Control and Prevention (CDC). During pregnancy. cdc.gov/pregnancy
- American Academy of Pediatrics / HealthyChildren.org. Prenatal health. healthychildren.org
This timeline is educational and is being brought under named clinical review; it is not a substitute for care from your own provider. Development and test timing are typical ranges, not rules for every pregnancy.