Week 8

At 8 weeks gestation, neurons start moving away from their birthplace in the brain to form the interconnected and layered cortex.⁵ Measurable brain activity has been recorded in human embryos as early as 45 days after conception, or 8 weeks and 3 days gestation, showing rhythmic electrical patterns rather than random neural firing.⁶ These early EEG signals suggest that neurons are already communicating and synchronizing their activity, even displaying patterns similar to sleep states seen later in development.⁷ As pregnancy progresses, brain activity becomes more complex, eventually allowing researchers to detect responses to light, sound, and other stimuli in the womb.

Neurons and muscles are connected as well. In week 8, the unborn baby can quickly move his arms or legs, as if startled.⁸ The unborn baby also moves away from a light touch to the nose or chin.⁹

As the unborn baby’s heart continues to grow at 8 weeks of pregnancy, it undergoes an extraordinary transformation into its familiar four-chambered shape. Thin walls called septa start dividing the heart into right and left sides. These walls develop from a special, jelly-like material that gives early structure to the heart. Meanwhile, the ventricles—the lower pumping chambers—grow rapidly, as new cells are added and muscle tissue thickens to prepare for a lifetime of work.¹⁵ One study in mice found that ventricles might increase in size nearly 100-fold!¹⁶

Though the atria, the upper blood-collecting chambers, appear symmetrical at first, each plays a unique role from the beginning. The right atrium absorbs early veins feeding from the body into the heart while the left atrium absorbs the pulmonary vein, which will carry oxygenated blood from the lungs after birth.¹⁷ These side-specific changes happen in a precise sequence to create a healthy and functional heart.

By the end of week 6 of pregnancy, two small areas of thickened skin appear on each side of the bulge of facial tissue called the frontonasal prominence. These areas are called nasal placodes, and they grow larger as the baby develops. During week 7, the middle of each nasal placode folds inwards to form depressions called nasal pits. These nasal pits eventually develop into the olfactory epithelium, the specialized tissue that makes up the inside of the nose and allows people to smell. The olfactory epithelium gives rise to olfactory receptor neurons, which start sending their axons towards the developing brain in week 8.³⁵ Also in week 8, the nasal cavity starts to connect to the back of the throat.³⁶

Folds of tissue surround the nasal pits to form the nostrils, in which human beings have millions of olfactory sensory neurons distributed along the surface!^{37 38} By 11 weeks gestation, the nose has developed its recognizable shape.³⁹ The nostrils remain plugged with thin tissue from 9 weeks to about 18 weeks’ gestation,^{40 41} but amniotic fluid still enters the nasal cavities when the baby starts swallowing around gestational week 12 .⁴²

At 8 weeks gestation, in the second month, the baby’s face, head and neck continue developing. Eyelids start forming,⁵¹ and pigments in the retina make the eyes more visible.⁵² Salivary glands start developing inside the mouth.⁵³ By the end of this week, key breathing and crying structures—the voicebox, trachea, and early lungs—have formed.⁵⁴

At the same time, the baby grows quickly—about 1 millimeter each day.^{55 56} Growth happens in bursts, especially in the arms, legs, and head. The hands develop one or two days before the feet. Tiny ridges called digital rays appear in the hands and feet, marking where fingers and toes will form.⁵⁷