8 min read · Pregnancy development from month 1 to birth
From a group of microscopic cells to a baby ready to meet you, pregnancy is one of the most remarkable periods of human development.
In just around 40 weeks, the brain and spinal cord begin to form, tiny limbs appear, organs develop, movement becomes stronger and your baby gradually prepares for life outside the womb.
This month-by-month guide gives you a simple overview of how your baby grows during pregnancy, approximately how big they are and what is developing along the way.
One important note before we begin: healthcare professionals usually measure pregnancy in weeks rather than months. Pregnancy is dated from the first day of your last menstrual period, which means the first approximately two weeks happen before conception. Month ranges are therefore only an easy way to visualise the journey.
Month 1: From one cell to a tiny embryo
Around weeks 1–4 · Approximately 2 mm by week 4 · About the size of a poppy seed
During the first two gestational weeks, conception has not usually happened yet. Your body is preparing for ovulation.
After fertilisation, the fertilised egg begins dividing rapidly while travelling towards the uterus. It develops into a cluster of cells called a blastocyst and eventually implants into the uterine lining.
Structures that will later help support your baby also begin forming, including the early placenta and amniotic sac.
By around week 4, the developing embryo may measure only about 2 millimetres. Tiny as it is, an extraordinary developmental process has already begun.
Month 2: The foundations of the body appear
Around weeks 5–8 · Approximately 16 mm by week 8 · About the size of a raspberry
The second month is one of the most important periods of early development.
The neural tube, which will become the brain and spinal cord, develops rapidly. Early structures of the heart form and begin rhythmic activity. Tiny buds appear where the arms and legs will grow.
The beginnings of the eyes, ears and mouth also appear, while the brain continues developing at remarkable speed.
By the end of this month, the embryo has recognisable early limbs and most major organ systems have begun developing, although they are still far from mature.
Month 3: The embryo becomes recognisably human
Around weeks 9–12 · Approximately 5.4 cm by week 12 · About the size of a plum
During the third month, the embryonic stage gives way to the fetal stage.
The face becomes increasingly defined. Fingers and toes separate, tiny nails begin forming and joints allow the arms and legs to move.
The bones start hardening and the major organs, muscles and body systems are now present and continue developing.
Your baby may already move inside the uterus, although these movements are still far too small for you to feel.
By approximately 12 weeks, the developing baby looks much more recognisably human, although the head remains large compared with the rest of the body.
Month 4: Movement becomes more coordinated
Around weeks 13–16 · Approximately 11.6 cm by week 16 · About the size of an avocado
Your baby is now growing quickly.
The nervous system continues developing and movements of the arms and legs become more coordinated. Hands can form fists, facial muscles begin making small expressions and sucking movements may appear.
Bones continue becoming stronger and the body slowly starts catching up with the still relatively large head.
The ears and hearing structures are also developing. Although you may not feel movement yet, your baby is already surprisingly active inside the uterus.
Month 5: You may feel the first little movements
Around weeks 17–20 · Approximately 25 cm by week 20 · Around the length of a banana
This is the month when pregnancy can suddenly begin to feel very different.
Many parents start noticing the first movements, sometimes described as bubbles, butterflies or gentle fluttering. These first noticeable movements are often called quickening, although exactly when they are felt varies considerably.
Your baby's hearing continues to develop, and sounds from inside and outside your body become part of their sensory environment.
A fine layer of hair called lanugo begins covering the skin, while a protective coating called vernix helps protect it in the amniotic fluid.
Your baby may suck their thumb, stretch, kick and move their arms and legs.
Month 6: Your baby hears, moves and practises
Around weeks 21–24 · Approximately 30 cm by week 24 · About the length of a corn cob
Your baby is becoming increasingly active.
Movements are more coordinated, muscles are strengthening and you may notice regular kicks or stretches. Your baby can hear sounds, including your heartbeat, breathing and voice.
The lungs continue developing and your baby makes breathing-like movements with amniotic fluid. Swallowing also becomes part of this constant preparation for life after birth.
The skin is still thin, and your baby has relatively little body fat compared with a newborn. From this point forward, growth and maturation become increasingly important.
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Month 7: Eyes open and the brain grows rapidly
Around weeks 25–28 · Approximately 37.6 cm by week 28 · About the size of an aubergine
Your baby's brain and nervous system are developing rapidly during this stage.
The eyelids can begin opening and closing, eyelashes are present and your baby may respond to light and sound.
The lungs begin producing surfactant, a substance that will eventually help the tiny air sacs remain open after birth, although the lungs still need considerable time to mature.
More fat is deposited beneath the skin, making your baby's body gradually look fuller and less wrinkled.
Month 8: Growing, listening and gaining weight
Around weeks 29–32 · Approximately 42.4 cm by week 32 · Around the length of a bunch of celery
By now, your baby is well formed, but there is still important development ahead.
The brain continues growing quickly and connections within the nervous system become more complex. Your baby can respond to sounds and may have increasingly noticeable periods of activity and rest.
Body fat continues accumulating, helping prepare your baby to regulate temperature after birth.
Most major organs are well developed by this stage, while the brain and lungs continue maturing during the remaining weeks.
Month 9: The final preparations for birth
Around weeks 33–40 · About 46 cm around week 35 and approximately 51 cm by week 40
During the final stretch, your baby continues gaining weight and body fat while the brain and lungs keep maturing.
The bones are well developed but remain flexible, particularly the bones of the skull. This flexibility is important during birth.
Many babies move into a head-down position during the final weeks, although timing and position vary.
By around 40 weeks, an average baby may measure approximately 51 cm from head to heel — roughly the length of a pumpkin — although healthy newborns come in many different sizes.
And then, after months of development you could not see but your body quietly supported every day, comes the moment you have been waiting for.
Your baby is ready to meet you.
Why are pregnancy sizes only approximate?
You may notice that different pregnancy apps compare the same week with different fruits or give slightly different measurements.
This is normal.
Fruit and vegetable comparisons are designed to make growth easier to imagine, not to provide a medical measurement. A plum, avocado or pumpkin can vary considerably in size.
Measurements also change during pregnancy. Earlier measurements are often taken from the top of the head to the bottom of the torso, while later estimates commonly describe head-to-heel length.
Ultrasound measurements and your healthcare team's assessment are much more meaningful than comparing your baby with an object at home.
Every pregnancy develops at its own pace
The milestones described here show the general pattern of typical fetal development. They are not a checklist your baby needs to complete on an exact day.
Growth rates, movement, measurements and pregnancy experiences vary between babies and between pregnancies.
Your midwife, obstetrician or other qualified healthcare professional can explain your baby's individual growth and development based on your pregnancy and medical care.
Frequently asked questions
How many weeks does pregnancy last?
A typical pregnancy is dated at around 40 weeks from the first day of the last menstrual period. Because conception usually occurs around two weeks later, this represents approximately 38 weeks from conception.
Why is pregnancy counted from before conception?
The exact date of conception is often difficult to determine. Healthcare professionals therefore calculate gestational age from the first day of the last menstrual period, which provides a consistent starting point.
When does an embryo become a fetus?
The transition happens near the end of the early embryonic period, during the first trimester. By around the third month, healthcare professionals generally refer to the developing baby as a fetus.
When can you usually feel the baby move?
Many pregnant people first notice movement during the second trimester, often somewhere around 17–20 weeks, although it may happen earlier or later.
When can a baby hear inside the womb?
The structures involved in hearing develop during the second trimester. As pregnancy progresses, babies become increasingly responsive to sounds including voices and noises from inside the parent's body.
Are fruit comparisons an accurate way to measure a baby?
No. Fruit comparisons are only a fun visual reference. Actual fetal growth is assessed using medical measurements and, when appropriate, ultrasound.
The Owloo approach
Pregnancy is filled with enormous changes that happen quietly, one tiny step at a time.
From a poppy-seed-sized beginning to the baby you will soon hold in your arms, every month tells another part of your story together.
Notice the changes. Keep the memories. Enjoy the journey before hello.
And when your baby arrives, the next chapter begins. Discover the Owloo Hello World Kit for babies from birth to 6 months, created to support calm visual, sensory and motor exploration during your baby's earliest months.
Written by Tina Krapez, Owloo Co-founder & COO
Tina Krapez is the Co-founder and COO of Owloo, where she helps create thoughtful play solutions and educational resources that support parents through their baby's early development. Her work focuses on turning developmental knowledge into simple, practical guidance families can use from pregnancy through their baby's first stages of discovery.
Sources and further reading
- NHS — Week-by-week guide to pregnancy
- MedlinePlus — Fetal development
- Cleveland Clinic — Fetal Development: Week-by-Week Stages of Pregnancy
