You've just found out you're pregnant. Or maybe you're halfway through and wondering what happens in the coming weeks. Wherever you are in your pregnancy — here's a complete overview of all 40 weeks.
How are pregnancy weeks counted?
Pregnancy is counted from the first day of your last menstrual period, even though fertilization occurs about two weeks later. This means you're technically not pregnant in weeks 1 and 2.
A full-term pregnancy is 40 weeks, divided into three trimesters:
- First trimester: Weeks 1–12
- Second trimester: Weeks 13–27
- Third trimester: Weeks 28–40
Use a due date calculator to work out your exact due date. You can also follow your entire pregnancy with a week-by-week timeline.
First trimester (weeks 1–12)
The first trimester is a period of enormous changes, even if you can't see anything on the outside yet.
Weeks 1–4: The beginning
Fertilization occurs in week 3. The egg implants in the uterine wall around week 4. Many women don't notice anything yet, but some experience light bleeding (implantation bleeding) or tender breasts.
The embryo at this point is smaller than a poppy seed.
Weeks 5–8: Organs form
The heart begins to beat as early as weeks 5–6. During these weeks, the foundations of all the major organs form — brain, lungs, liver, and kidneys.
Many experience:
- Nausea (morning sickness that can last all day)
- Fatigue
- Frequent urination
- Tender breasts
Up to 80 percent of pregnant women experience nausea in the first trimester. It's a good sign that hormone levels are as they should be.
Weeks 9–12: The fetus takes shape
By week 12, the fetus is about 6 cm long. Fingers and toes are fully formed, and the fetus can move its arms and legs — though you won't feel it yet.
An ultrasound in weeks 11–14 is the first routine prenatal scan. It checks the baby's development and estimates your due date.
Second trimester (weeks 13–27)
Many describe the second trimester as the best period. Nausea eases, energy returns, and your belly starts to show.
Weeks 13–16: Energy returns
The placenta now takes over hormone production. For many, this means nausea subsides and energy levels increase. The fetus begins swallowing amniotic fluid and the kidney system starts up.
Weeks 17–20: You feel the first kicks
Most first-time parents feel movement for the first time between weeks 18 and 22. It can feel like bubbles, flutters, or gentle taps. Those who have been pregnant before often feel it earlier.
The anatomy scan at around weeks 18–20 is the second routine scan. The fetus's anatomy is thoroughly checked, and many choose to find out the sex.
Weeks 21–27: Rapid growth
The fetus grows quickly. By week 24, it weighs about 600 grams and is around 30 cm long. The lungs develop, and the fetus begins to respond to sound.
You may experience:
- Back problems
- Heartburn
- Pelvic girdle pain
- Stretch marks
- Swelling in hands and feet
Follow your baby's development week by week with a pregnancy calendar featuring images and information. You can also add personal notes to create your own pregnancy story.
Third trimester (weeks 28–40)
The home stretch. Your baby is growing big, and your body is preparing for birth.
Weeks 28–32: Baby matures
The lungs develop surfactant, a substance that enables them to function outside the womb. Your baby now has distinct sleep and wake periods, and you can feel the kicks clearly.
Blood tests and possibly a glucose tolerance test are done during this period.
Weeks 33–36: Preparation
The baby is gaining fat and your body is starting to prepare. Braxton Hicks contractions (practice contractions) are common. Most babies turn head-down during this period.
Now is the time to:
- Pack your hospital bag
- Write your birth plan
- Enroll in a childbirth preparation class
Weeks 37–40: Ready for birth
The baby is full-term from week 37. In the final weeks, it gains about 200 grams per week. By week 40, the average baby weighs around 3,500 grams.
Signs that birth is approaching:
- The mucus plug is released
- Regular contractions
- Water breaking
Only about 4 percent of women give birth on their exact due date. Most give birth between weeks 38 and 42. Use a contraction timer to keep track when things start happening.
Important prenatal checks throughout pregnancy
Standard prenatal care typically follows this schedule:
| Week | Check-up |
|---|---|
| 8–12 | First prenatal appointment |
| 11–14 | Early ultrasound (optional) |
| 18–20 | Anatomy scan |
| 24 | Check-up with blood pressure, urine, fundal height |
| 28 | Check-up, blood tests |
| 32 | Check-up |
| 36 | Check-up, birth discussion |
| 38 | Check-up |
| 40 | Check-up, possible assessment for induction |
| 41+ | More frequent monitoring |
First-time parents are typically offered more check-ups than those who have been pregnant before. Discuss your individual needs with your midwife or OB.
Common discomforts trimester by trimester
First trimester
- Nausea and vomiting
- Fatigue
- Tender breasts
- Mood swings
Second trimester
- Heartburn and acid reflux
- Constipation
- Pelvic girdle pain
- Nosebleeds and nasal congestion
Third trimester
- Sleep problems
- Pelvic pain
- Shortness of breath
- Swelling in hands and feet
- Headaches
Frequently asked questions
How accurate is the due date calculation?
A due date based on ultrasound at weeks 11–14 is the most accurate, with a margin of error of plus or minus 5–7 days. Calculation from last menstrual period is less precise.
When should I tell people I'm pregnant?
That's entirely up to you. Many wait until after week 12, when the risk of miscarriage is lowest. Others tell people right away. There are no rules.
Is it dangerous to exercise during pregnancy?
No, quite the opposite. WHO recommends at least 150 minutes of moderate physical activity per week throughout pregnancy. Adapt the intensity and listen to your body.
Sources
- WHO — Antenatal care guidelines
- American College of Obstetricians and Gynecologists (ACOG) — Prenatal development
- Mayo Clinic — Fetal development week by week