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First Trimester (Weeks 1–12): Foundations of Life
Your baby’s story starts even before you realise you’re pregnant. During these early weeks, the groundwork for every organ and system is laid down.
Weeks 1–4: Fertilisation and Implantation: During the first two weeks, your body is preparing for ovulation and fertilisation. Around week 3, the sperm meets the egg, forming a zygote, a single cell that carries half of your DNA and half of your partner’s. By week 4, this tiny ball of cells, now called a blastocyst, implants itself into your uterine lining. That’s when pregnancy officially begins. The placenta begins to form, setting up the nutrient exchange system between you and your baby.
Weeks 5–6: The Heart Starts Beating: Around week 5, your baby is the size of a sesame seed. Major organs, including the brain, spinal cord, and heart, start developing. By week 6, that heart begins to beat, visible on an ultrasound as a tiny flicker. It’s one of the earliest milestones most parents cherish.
Weeks 7–8: Forming a Miniature Body: Facial features, limb buds, and the umbilical cord form by week 8. The baby, now called an embryo, measures around 1.5 cm. Neural development accelerates, and the digestive tract and sensory nerves begin to take shape.
Weeks 9–12: From Embryo to Fetus: At week 9, your baby officially becomes a fetus. The head grows quickly as the brain develops billions of neurons. By week 12, internal organs like the kidneys and liver start functioning, bones begin to harden, and external genitalia form, though they’re not yet visible on scans. Your baby is roughly the size of a lime by the end of the first trimester.
These first 12 weeks lay the foundation for everything that follows. Even though your baby still weighs less than 30 grams, almost every organ system already exists.
Second Trimester (Weeks 13–27): Growth, Movement, and Senses
Often called the “golden period,” the second trimester brings relief from early pregnancy fatigue and visible signs of your baby’s rapid growth.
Weeks 13–16: Stretching, Flexing, and Growing: Your baby’s muscles strengthen, allowing small movements, though you may not feel them yet. By week 14, facial muscles start working, and your baby may make tiny expressions. At week 16, the baby is about 10–12 cm long, with bones visible on ultrasound.
Weeks 17–20: You Feel the First Kicks: By week 18, the skeleton hardens, and you may feel the first fluttering sensations known as quickening. The baby’s hearing begins to develop, and fingerprints appear. The skin is thin and covered by a protective waxy layer called vernix caseosa.
Weeks 21–24: Sensory Awakening: Your baby’s movements become more coordinated. By week 22, taste buds are active, and the baby can sense flavours in the amniotic fluid depending on your diet. By week 24, the lungs begin forming alveoli, the tiny air sacs crucial for breathing after birth. At this stage, your baby weighs around 600 grams and measures nearly 30 cm long.
Weeks 25–27: Stronger Reflexes and Brain Growth: The baby’s brain develops sleep and wake cycles. Eyelids open around week 26, and the baby may respond to your voice, light, and touch. Fat stores begin to accumulate under the skin, smoothing the earlier wrinkled appearance.
During these middle months, your baby transforms from a delicate fetus into an active, responsive little person. You’ll notice patterns of kicks and rolls, a comforting reminder that growth is on track.
Third Trimester (Weeks 28–40): Maturity and Readiness
The final stretch is about gaining strength, fat, and lung maturity. Your baby’s body prepares for life outside the womb.
Weeks 28–32: Eyes, Brain, and Body Fat Develop: Your baby can now blink, and the pupils respond to light. The brain’s surface folds to form grooves, increasing capacity for learning and memory. Fat layers form, helping regulate body temperature after birth. By week 32, the baby weighs around 1.8 kilograms.
Weeks 33–36: Coordination and Positioning: The baby’s bones are fully formed but still soft. Around week 34, many babies turn head-down in preparation for birth. The nervous system and immune system mature, and lungs produce surfactant, a substance that prevents the air sacs from collapsing after breathing begins.
Weeks 37–40: Full-Term and Ready for the World: At 37 weeks, your baby is considered full-term. The average weight is between 2.8–3.2 kilograms, and the length ranges from 48–52 cm. Movements may feel different now, more like stretches than kicks, because space is limited. Vernix and fine hair (lanugo) gradually shed, and your baby’s lungs, liver, and brain continue final growth until birth.
By the final weeks, your body and your baby are synchronised for one shared goal, safe arrival. Every contraction, every ounce of weight gain, and every heartbeat now serves that purpose.
How Doctors Track Your Baby’s Growth
You’ll have regular antenatal check-ups to monitor development. Doctors use several tools:
- Ultrasound scans: Measure head circumference, abdominal girth, and femur length.
- Fundal height: The distance from your pubic bone to the top of your uterus helps estimate growth.
- Doppler tests: Assess blood flow in the umbilical cord and placenta.
- Weight tracking: Steady maternal weight gain supports foetal growth.
Factors That Influence Foetal Growth
While genetics play a role, your baby’s growth also depends on your overall health and lifestyle.
Key factors include:
- Placental health: A well-functioning placenta ensures oxygen and nutrient supply.
- Nutrition: Adequate protein, iron, folate, calcium, and omega-3s support growth.
- Maternal health: Anaemia, thyroid disorders, or hypertension can affect development.
- Lifestyle habits: Smoking, alcohol, or poor sleep can restrict growth.
- Emotional stress: Chronic stress hormones can influence blood flow to the uterus.
When to Call Your Doctor
While growth variation is normal, certain signs need medical attention:
- Noticeably reduced baby movements after week 28
- Unusual drop in fundal height
- Persistent abdominal pain or fluid leakage
- High blood pressure, swelling, or dizziness
Bonding With Your Baby as They Grow
Even before birth, your baby responds to sound, touch, and emotion. Talking, singing, or gently massaging your belly builds a subtle connection. Studies suggest that babies recognise their mother’s voice and heartbeat patterns after birth.
Culturally, many Indian families practice garbh sanskar, mindful nurturing through soothing music, prayers, and storytelling. While scientific evidence varies, these rituals can help you feel calm and connected, indirectly supporting your baby’s well-being.
Every week of pregnancy represents a miracle in progress. While it’s easy to worry about numbers on a scan or belly size comparisons, remember that growth happens in rhythms, not races. Staying nourished, rested, and emotionally balanced gives your baby the best environment to thrive. As your pregnancy advances, each kick, heartbeat, and ultrasound image marks another week closer to meeting your baby, fully grown in every sense that matters.
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FAQs on Your Baby’s Growth Week by Week: What’s Happening Inside Your Belly
- How accurate are foetal growth scans?
Ultrasound measurements have a small margin of error, usually ±10%. Doctors look for trends over time rather than single numbers. - Can my diet affect my baby’s size?
Yes. A balanced diet with enough protein, iron, and vitamins supports proper growth. Extremely high or low calorie intake can affect weight gain. - What if my baby is measuring small on ultrasound?
Sometimes genetics or maternal build explain a smaller size. Your doctor may order Doppler or biophysical profile tests to ensure normal function. - How much weight does a baby gain in the last trimester?
Babies typically gain about 200–250 grams per week after 30 weeks, accounting for most of their final birth weight.