The Human Fetus: Week 8 to Birth Timeline

The Human Fetus: Week 8 to Birth 28

Week 8 The fetal period begins Fetal development All major organs are in place. About 3 cm from crown to rump and 8 grams, the embryo is now called a fetus.
Week 8 The liver makes the blood Fetal development Blood formation moves from the yolk sac to the liver, which stays the main source until late pregnancy.
Week 9 First movements Fetal development Twitches, stretches and breathing-like movements begin, far too small for the mother to feel.
Week 10 Intestines return to the abdomen Fetal development The gut loop that bulged into the umbilical cord at week 6 moves back into the belly and rotates into place.
Week 10 Kidneys start making urine Fetal development The fetus passes urine into the amniotic fluid, which becomes a large part of that fluid.
Week 11 Swallowing begins Fetal development The fetus swallows amniotic fluid, practice for feeding and important for the growth of the gut.
Week 12 Boy or girl: the genitals take shape Fetal development The external genitals, identical until now, develop as male or female.
Week 13 Fingerprints begin to form Fetal development Ridges develop on the fingertips. The pattern, unique even between identical twins, is fixed within a few weeks.
Week 14 Lanugo appears Fetal development Fine, downy hair starts to cover the body. It helps hold the protective vernix in place.
Week 15 Nerve cells begin to connect Fetal development Synapses form in large numbers, the start of the wiring of the brain.
Week 18 Halfway: about 300 grams Fetal development At the midpoint of pregnancy the fetus is about 25 cm long from head to heel.
Week 18 The mother feels the first kicks Fetal development Quickening: movements become strong enough to feel, usually around weeks 18–19, earlier in a second pregnancy.
Week 18 Vernix covers the skin Fetal development A white, waxy coating protects the skin from the amniotic fluid.
Week 22 Threshold of viability Fetal development With intensive care, about half of babies born now survive. Each further week raises the odds sharply.
Week 22 Lungs start making surfactant Fetal development A soapy substance that keeps the air sacs from collapsing. Without enough of it, breathing after birth is hard.
Week 22 Thalamus connects to the cortex Fetal development Pathways carrying signals from the senses reach the outer layer of the brain.
Week 24 Hearing is complete Fetal development The inner ear is fully formed. The fetus reacts to sound and learns the rhythm of the mother's voice.
Week 25 The eyes open Fetal development The eyelids, fused shut since week 8, open. The eyes start to react to light.
Week 26 About one kilogram Fetal development Around 35 cm long. From now on the fetus mainly gains weight and fat.
Week 28 The brain surface folds Fetal development The smooth cortex develops its grooves and ridges, packing more brain into the skull.
Week 28 REM sleep and dreaming Fetal development Sleep cycles with rapid eye movement appear. The fetus spends most of its time asleep.
Week 28 Bone marrow takes over the blood Fetal development The bone marrow becomes the main site of blood formation, as it will be for life.
Week 30 Fat builds up under the skin Fetal development The fetus rounds out. Fat will keep the newborn warm.
Week 32 Turns head down Fetal development Most babies settle into the head-first position for birth.
Week 33 Lanugo is shed Fetal development Most of the fine body hair falls out into the amniotic fluid.
Week 34 Lungs mature Fetal development The lungs now make enough surfactant to breathe without help in most babies.
Week 35 Full term begins Fetal development From here a birth is no longer premature. Growth slows down.
Week 38 Birth Fetal development On average 38 weeks after fertilisation. About 50 cm long and 3.5 kg.