From the moment egg and sperm meet to the moment you hold your baby in your arms, an extraordinary transformation takes place. In just 40 weeks, a single cell becomes a fully formed human being. Here is what happens in each trimester — and what you can expect along the way.
First Trimester: Weeks 1–12
The first 12 weeks are the most intensive period of fetal development. Every organ system is established, and the foundation for the rest of the pregnancy is laid.
Weeks 1–4: Conception and Implantation
Pregnancy is counted from the first day of your last period, even though conception usually happens around week 2.
- Week 2: Ovulation and fertilization. The egg and sperm fuse to form a zygote
- Week 3: The fertilized egg divides rapidly as it travels down the fallopian tube
- Week 4: Implantation in the uterine wall. Some women notice light bleeding (implantation bleeding)
Weeks 5–8: Organs Take Shape
Enormous changes are happening. The developing organism is called an embryo at this stage.
- Weeks 5–6: The heart begins to beat — first as a simple pulse, then with a regular rhythm. The heartbeat can often be seen on ultrasound from week 6–7
- Week 7: Arms and legs begin as tiny buds. The brain develops rapidly
- Week 8: All major organ systems are established. Fingers and toes begin to form. The embryo is about 1.5 cm long
Weeks 3–8 are the most sensitive period of fetal development. Organs are forming, and the embryo is especially vulnerable to outside influences such as alcohol, medications, and infections. The WHO and AAP both recommend avoiding alcohol entirely from the moment you start trying to conceive.
Weeks 9–12: The Fetus Becomes Recognizable
From week 9, it is referred to as a fetus (no longer an embryo).
- Weeks 9–10: Facial features form. Eyelids, ears, and nose become visible. The fetus starts moving, though you cannot feel it yet
- Weeks 11–12: All organs are in place and beginning to mature. Fingers and toes are distinct. The fetus is about 5–6 cm long and weighs around 14 grams
First ultrasound (weeks 11–14): Most providers offer a first-trimester ultrasound between weeks 11 and 14. This confirms the due date, number of fetuses, and measures the nuchal translucency.
Many people experience nausea, fatigue, and breast tenderness in the first trimester. This is completely normal and caused by hormonal changes. For most, symptoms ease toward the end of weeks 12–14.
Second Trimester: Weeks 13–27
The second trimester is often called the "golden period." Nausea usually subsides, energy returns, and the bump starts to show.
Weeks 13–16: Growth and Movement
- Weeks 13–14: The fetus can suck its thumb and swallow amniotic fluid. The skeleton begins to ossify
- Weeks 15–16: The fetus is about 10–12 cm long. People who have given birth before may feel the first movements already — a faint tickle or bubbling sensation
Weeks 17–20: You Feel Kicks!
- Weeks 17–19: Most first-time parents feel movement for the first time between weeks 18–22. It initially feels like small bubbles or butterflies in the belly
- Weeks 18–19: The mid-pregnancy anatomy scan is typically offered around this time. The fetus's organs, growth, and placenta position are examined. The sex can usually be determined now
- Week 20: Halfway! The fetus is about 25 cm long and weighs around 300 grams
The anatomy scan (also called the anomaly scan) is offered to all pregnant people and involves a thorough examination of the fetus's anatomy. You can choose whether you want to know the sex.
Weeks 21–27: Senses Awaken
- Weeks 21–24: The fetus develops hearing and can respond to sounds. It recognizes its mother's voice. Eyebrows and eyelashes form
- Weeks 25–27: The fetus opens its eyes for the first time. The lungs continue to develop but are not yet mature. By the end of the second trimester, the fetus weighs about 800–1,000 grams
Size Chart — Second Trimester
| Week | Length (approx.) | Weight (approx.) | Comparison |
|---|---|---|---|
| 14 | 9 cm | 45 g | Lemon |
| 18 | 14 cm | 200 g | Bell pepper |
| 22 | 28 cm | 430 g | Papaya |
| 26 | 35 cm | 760 g | Cauliflower |
Third Trimester: Weeks 28–40
The final stretch. The fetus gains weight, the lungs mature, and everything prepares for life outside the womb.
Weeks 28–32: Weight Gain and Maturation
- Weeks 28–30: The brain develops rapidly, forming new neural connections. The fetus can dream (REM sleep has been confirmed). It gains about 200 grams per week
- Weeks 31–32: The lungs produce surfactant — a substance that keeps the air sacs open after birth. The fetus often turns head-down
Weeks 33–36: Almost Ready
- Weeks 33–34: The skeleton is nearly fully ossified, except for the skull, which remains soft to ease delivery
- Weeks 35–36: The fetus weighs about 2.5 kg and is around 46 cm long. Antibodies from the mother pass through the placenta. Most babies are now in a head-down position
From week 28, it is a good idea to get familiar with your baby's movement pattern. Your baby has its own rhythm that you will learn to recognize. If you notice a clear reduction in movement or a change in the pattern, contact your midwife or maternity unit.
Weeks 37–40: Due Date Approaches
- Week 37: The fetus is now considered full-term. The lungs are usually mature
- Weeks 38–39: The baby gains fat that gives it that round, chubby look. Meconium (the first bowel movement) forms in the intestines
- Week 40: Due date! The baby typically weighs 3–4 kg and is 48–52 cm long. Only about 5% of babies are born on their exact due date
Average Birth Weight
| Category | Weight |
|---|---|
| Low birth weight | Under 2,500 g |
| Normal birth weight | 2,500–4,500 g |
| Average (global) | Approx. 3,300–3,500 g |
| High birth weight | Over 4,500 g |
Prenatal Care Schedule
Most healthcare providers follow a schedule of regular prenatal checkups. Here is a typical overview:
| Week | Appointment | What is checked |
|---|---|---|
| 8–12 | First prenatal visit | Health history, blood tests, baseline measurements |
| 11–13 | First-trimester ultrasound | Due date, number of fetuses, nuchal translucency |
| 17–19 | Anatomy scan | Fetal organs and anatomy |
| 24 | Prenatal checkup | Blood pressure, urine, fundal height |
| 28 | Prenatal checkup | Blood tests, growth, movement |
| 32 | Prenatal checkup | Position, growth, blood pressure |
| 36 | Prenatal checkup | Birth preparation, position |
| 38 | Prenatal checkup | Position, signs of labor |
| 40 | Due date check | Assessment, possible induction discussion |
| 41+ | Overdue monitoring | NST, amniotic fluid assessment |
The prenatal care schedule may vary depending on your provider and your specific situation. First-time parents often have more appointments than those who have given birth before. Talk to your midwife or OB about what is right for you.
When to Contact Your Provider
Between appointments, there are symptoms you should always contact your healthcare provider about:
- Vaginal bleeding — especially in the second and third trimester
- Severe abdominal pain that does not go away
- Noticeably reduced fetal movement — fewer movements than usual
- Sudden, severe headache with visual disturbances
- Sudden swelling of the face and hands
- Fever above 38°C (100.4°F)
- Fluid leaking from the vagina — may indicate that your waters have broken
Trust your instincts. If something feels wrong, call your midwife or maternity unit. You are never overreacting — healthcare providers would always rather check once too often than miss something important.
Frequently Asked Questions
When does the fetus start to hear?
Hearing develops gradually from about week 18, and from weeks 24–25 the fetus can respond to sounds from outside the womb. Research shows that newborns recognize voices and music they heard in utero. Feel free to talk and sing to your baby from the second trimester onward.
Is it normal for the baby to move less toward the end?
The movement pattern changes because the baby has less room, but the number of movements should not decrease noticeably. You should still feel your baby being active. If you notice clearly fewer movements, contact your maternity unit for a checkup.
What is the difference between an embryo and a fetus?
Until week 8, it is called an embryo — this is the phase when all basic organs and structures are formed. From week 9 it is called a fetus, and the focus shifts from organ formation to growth and maturation of the organs that are already in place.