Fetal Growth Month by Month: A Complete Guide
Short answer: A baby grows from a single cell to a full-term newborn over 9 months: by the third month all major organs are formed, in the fifth you feel movement and can learn the sex, by 24 weeks the baby is viable, and weight gain accelerates in the seventh and eighth months to reach 2.5-4 kilograms at birth.
Nine months of consecutive miracles: from a single microscopic cell to a complete baby weighing three kilograms who breathes and cries. Understanding what happens inside your womb month by month gives you deep reassurance and lets you follow your pregnancy consciously and enjoy every stage.
In this article, we summarize the key milestones of fetal development in each month of pregnancy, with approximate size and weight so you can picture your little one, plus a reference table of figures at the end.
Quick takeaways
- The first trimester (through week 13) is the foundation stage: all major organs form.
- You feel your baby move between weeks 16 and 20, and the sex can be seen on ultrasound from the fifth month.
- From week 24, the baby is viable outside the womb with intensive care.
- The fastest weight gain happens in the seventh and eighth months (about 200 grams per week).
- Normal birth weight is between 2.5 and 4 kilograms; small differences from the average are normal.
Months One and Two: The Miraculous Beginning
Month one: fertilization happens in the fallopian tube, then the fertilized egg implants in the uterine wall after 6-12 days. Cells begin dividing and differentiating: some become the embryo and some become the placenta. By the end of week four, the embryo is the size of a poppy seed (about 2 mm), and the neural tube (which will become the brain and spinal cord) and the primitive heart begin forming. At this stage, you may not know you are pregnant yet.
Month two (weeks 5-8): the fastest development phase of all. The heart starts beating around week six, arm and leg buds form, then fingers, and the first facial features, eyes, and ears take shape. By the end of the second month, the baby is about 3 cm long (the size of a kidney bean) and is officially called a fetus after having been an embryo. This is the most sensitive stage, so avoiding medications, smoking, and alcohol completely is advised.
Months Three and Four: Formation Complete
Month three (weeks 9-12): all major organs are complete and start working together. Fingernails form, the baby starts moving its limbs (you will not feel it yet), and the reproductive organs develop. By the end of week 12, the baby is about 5.5 cm long and weighs just 14 grams. With this month over, the most sensitive phase ends and the risk of miscarriage drops sharply, a moment when many mothers breathe a sigh of relief.
Month four (weeks 13-16): the baby is about 15 cm long (the size of an avocado) and weighs 140 grams. It begins hearing muffled sounds from the outside world, its unique fingerprints form, and its body is covered in fine downy hair. Near the end of this month, you may feel the first light fluttering like bubbles, especially if this is not your first pregnancy.
Months Five and Six: Movement and Life
Month five (weeks 17-20): you clearly feel kicks now, and the baby develops a regular sleep-and-wake cycle. The baby's sex can be seen on the detailed ultrasound (18-22 weeks) with accuracy exceeding 95%. The baby is about 25 cm long and weighs 300 grams (the size of a banana). Head hair starts growing, and the sense of taste develops.
Month six (weeks 21-24): the baby opens its eyes and responds to loud sounds, and fingerprints and footprints are complete. The biggest milestone: from week 24, the baby is viable outside the womb with intensive neonatal care, and survival chances rise with every additional week. The baby weighs about 660 grams and is 35 cm long.
Months Seven and Eight: Rapid Growth
Month seven (weeks 25-28): brain growth accelerates dramatically and the lungs develop in preparation for breathing. The baby's weight increases fast, about 200 grams per week. The baby weighs about 1.2 kilograms and is 39 cm long. It responds clearly to light and sound, and you may notice a regular pattern of sleep and movement. By the end of this month, the lungs are nearly complete, though they keep maturing.
Month eight (weeks 29-32): rapid weight gain continues, and the baby reaches about 2.1 kilograms and 45 cm. A layer of fat develops under the skin, making it less wrinkled. Most babies settle head-down in preparation for birth. Almost all organs are complete except the lungs and brain, which keep maturing until the end.
Month Nine: Getting Ready to Meet You
Month nine (weeks 33-40): growth is complete and the lungs are ready for breathing air. The baby's head gradually descends into the pelvis in preparation for birth (lightening), and you may feel more pressure on your bladder and easier breathing. Normal birth weight ranges between 2.5 and 4 kilograms, and length is about 51 cm.
In the final weeks, the baby gradually sheds the fine downy hair, accumulates fat to become chubby, and its immune system matures thanks to antibodies delivered through the placenta. Birth between weeks 37 and 42 is considered on time; before 37 is premature and after 42 is post-term, needing special follow-up.
Approximate Weight and Length Table, Month by Month
| Month | Weeks | Approx. weight | Approx. length | Size of |
|---|---|---|---|---|
| First | 1-4 | Less than a gram | 2 mm | A poppy seed |
| Second | 5-8 | 1 gram | 3 cm | A kidney bean |
| Third | 9-12 | 14 grams | 5.5 cm | A lemon |
| Fourth | 13-16 | 140 grams | 13 cm | An avocado |
| Fifth | 17-20 | 300 grams | 16 cm | A banana |
| Sixth | 21-24 | 660 grams | 35 cm | An ear of corn |
| Seventh | 25-28 | 1.2 kg | 39 cm | A cauliflower |
| Eighth | 29-32 | 2.1 kg | 45 cm | A coconut |
| Ninth | 33-40 | 3.3 kg | 51 cm | A small watermelon |
These figures are global averages, and your baby may differ slightly from them, which is completely normal; do not worry about small differences in ultrasound measurements. What matters is the continuous growth curve, not a single number. If the doctor notices slowed growth, you will be followed with regular ultrasounds, and most of these cases end with a healthy delivery.
How to Support Your Baby's Growth
Healthy fetal growth depends on three things in your hands: balanced nutrition (daily protein, iron, folic acid, calcium), avoiding toxins (smoking, alcohol, and medications without medical advice), and regular medical follow-up that catches any slowdown early.
Rest and enough sleep also support growth; growth hormone is mainly secreted during deep sleep. Avoid chronic stress as much as possible; daily walks, talking to your baby, and touching your belly are all simple things that make a real difference in your journey together.
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Open the calculator →Frequently asked questions
In which month can I know my baby's sex?
Usually in the fifth month (18-22 weeks) with a detailed ultrasound, with accuracy exceeding 95%, and sometimes from the end of the fourth month if the position is clear.
When does the baby start moving?
You feel the first fluttering between 16 and 20 weeks (the fourth and fifth months), growing stronger and more regular as pregnancy progresses. In a first pregnancy, you may be slower to recognize it.
Is the baby's ultrasound size accurate?
It is an estimate with a margin of error that grows in the later months, reaching 10-15% in weight estimates; do not worry about small differences from the tables.
When are the baby's lungs fully developed?
The lungs mature gradually and are nearly complete by 37 weeks (the start of the ninth month), which is why birth before that is considered premature.
When can the baby hear sounds?
It starts hearing muffled sounds from the fourth month and responds to loud sounds and music from the sixth month.
Is a 2.5 kg birth weight normal?
Yes; the minimum for a normal full-term birth weight is 2.5 kilograms, and the global average is about 3.3 kilograms.