All body systems appear and begin to function during the first 8 weeks following fertilisation
(3–5 weeks since LMP*)
At the moment of fertilisation a baby's unique genome (DNA code) is formed, determining eye colour, hair colour and sex. By 3 weeks the brain, spinal cord and heart have all started to develop. The baby's heart starts to beat at the end of the 3rd week.
(6 weeks since LMP)
The heart is pumping the baby's own blood to his or her brain and body. All four chambers of the heart are present and more than one million heartbeats have occurred. The brain is developing rapidly. Arms and legs begin to grow.
(7 weeks since LMP)
The baby reflexively turns away in response to touch on the face at 5.5 weeks. Tiny fingers are beginning to form.
(8 weeks since LMP)
Brain waves have been recorded from 6 weeks, 2 days. Head to heel length 1.6 cm.
(9 weeks since LMP)
The hands start to move and the neck can turn. The baby has started to hiccup! Girls now have ovaries and boys have testes. Their heart rate peaks at about 170 beats per minute and will gradually slow down until birth. Electrical recordings of the heart at 7.5 weeks are very similar to the ECG tracing of a newborn. The heart is nearly fully formed. Head to heel length 2.2 cm.
(10 weeks since LMP)
Fingers and toes are now fully formed, and several hundred muscles are present. Most babies start to show first signs of right or left-handedness. The baby's overall appearance and most internal structures closely resemble a newborn.
Experts estimate the 8-week baby possesses approximately 90% of the 4,500 body parts found in adults, yet incredibly this highly complex little embryo weighs just under 3 grams. Head to heel length 4.3 cm.
From 8 weeks, 1 day the medical term for your developing baby is not an 'embryo' but a 'fetus', which is Latin for 'little one' or 'unborn offspring'.
(11 weeks since LMP)
The baby periodically sighs and stretches and has started thumb sucking and yawning.
The face, palms of the hands and soles of the feet are sensitive to light touch.
Girls’ ovaries now contain reproductive cells which will give rise to eggs later in life. Also in girls, the uterus is now present.
(12 weeks since LMP)
The baby's unique fingerprints start to form. Fingernails and toenails start to grow. Bones are hardening in many locations. The baby now weighs 28 grams. Head to heel length 7.5 cm.
(13 weeks since LMP)
The lips and nose are now fully formed and the baby can make complex facial expressions.
(14 weeks since LMP)
The baby now has taste buds. Their arms are finally in proportion to body size. Head to heel length 12.5 cm.
(15 weeks since LMP)
The baby responds to touch. Teeth have started to develop.
(16 weeks since LMP)
You may begin to feel the baby move. They weigh 113 grams. Head to heel length 17.5 cm.
(17 weeks since LMP)
The baby begins storing energy in the form of baby fat.
The production of a variety of digestive enzymes is well underway and blood cell formation moves to its permanent location inside the bone marrow.
(18 weeks since LMP)
The formation of the breathing passages, called the bronchial tree, is complete. The baby weighs 170 grams. Head to heel length 20 cm.
(19 weeks since LMP)
More than 20 million heartbeats have occurred.
(20 weeks since LMP)
Between 18 and 21 weeks rapid eye movement begins—similar to that seen when a child or adult is dreaming. The baby weighs 255 grams. Head to heel length 25 cm.
(21 weeks since LMP)
Breathing patterns, body movements and heart rate begin to follow daily schedules.
(22 weeks since LMP)
Hair has started to grow on the baby's head.
The sense of hearing begins to function and the baby starts responding to various sounds. The cochlea, the organ of hearing, reaches adult size. All skin layers and structures are complete.
The baby weighs 450 grams. Head to heel length 28 cm.
(23 weeks since LMP)
If born prematurely from this point on, survival is possible with specialised medical care.
(24 weeks since LMP)
More than 30 million heartbeats have occurred. The baby weighs 560 grams. Head to heel length 30.5 cm.
(25 weeks since LMP)
Breathing motions may occur up to 44 times per minute.
(26 weeks since LMP)
Loud noises may startle the baby causing increased movement, heart rate and swallowing.
The baby weighs 900 grams. Head to heel length 35 cm.
(27 weeks since LMP)
The baby now has the ability to taste. Their thigh and foot bones are each 5 cm.
(28 weeks since LMP)
The eyes can produce tears and the sense of smell is functioning. The baby weighs 1.1 kg. Head to heel length 38 cm.
(29 weeks since LMP)
Pupils of the eyes react to light.
(30 weeks since LMP)
The baby weighs 1.5 kg. Head to heel length 40.5 cm.
(31 weeks since LMP)
Wrinkles in the skin are starting to disappear as the baby is putting on more body fat. By now more than 40 million heartbeats have occurred.
(32 weeks since LMP)
Breathing movements occur up to 40% of the time. The baby weighs 1.8 kg. Head to heel length 43 cm.
(33–34 weeks since LMP)
By 32 weeks true alveoli, or 'air pocket' cells, begin developing in the lungs. The baby weighs 2.27 kg. Head to heel length 46 cm.
(35–36 weeks since LMP)
By 34 weeks the baby weighs 2.6 kg. Head to heel length 47 cm.
(37–38 weeks since LMP)
By 35 weeks the baby now has a firm hand grip and the heart has beaten more than 50 million times. By 36 weeks the baby now weighs 3 kg. Head to heel length 48 cm.
(39–40 weeks since LMP)
The baby continues to grow and put on weight until birth.
Labour is initiated by the baby, ideally around 38 weeks, leading to childbirth.
At full-term birth babies typically weigh 2.7–4 kg. Head to heel length between 46–53 cm.
In preparation for life after birth, many daily activities start more than 30 weeks before birth. For example, hiccups, swallowing, yawning, jaw movement, hearing and sensation.
Prenatal development descriptions are taken from The Endowment for Human Development, measurements are approximate.
*Medically, pregnancy is dated (as day 0) since the 1st day of the last menstrual period (LMP) to help calculate a due date (full-term usually 40 weeks). This is 2 weeks greater than the baby's prenatal age, dated from fertilisation (full-term usually 38 weeks).