The Embryonic Period: The First 8 Weeks
With fertilization, your development begins. At this moment, your father’s sperm enters your mother’s oocyte to create a one-cell embryo called a zygote. That’s you! Your genetic make-up is complete. As a zygote, you divide rapidly into many cells. Between 6 and 12 days, you become a blastocyst, moving through the fallopian tube and attaching to the inner wall of your mother’s uterine wall. Your placenta and umbilical cord begin to form.
By 2 weeks, 4 days your neural plate emerges.
The development of your brain and spinal cord begin as your neural tube appears. Your blood cells and blood vessels form. By three weeks, 1 day your heart begins to beat in your developing circulatory system.
By 4 weeks, your cerebellum is developing and by 4 ½ weeks, your cerebral hemispheres appear. Brainwaves have been recorded at 6 weeks, 2 days.
Between 5 ½ and 6 weeks, movement begins. You first move your head and twist your trunk. This is essential for the normal development of your bones and joints.
Your fingers begin to separate by 6 ½ weeks. By 7 ½ weeks, they are completely free. Your hands can touch each other and so do the feet.
The Fetal Period: 8 Weeks to Birth
By 8 weeks, handedness is evident. There is a 75% chance that you exhibit right-hand dominance.
At 7 ½ weeks, your heart has 4 chambers and beats about 170 times per minute. You are a little more than an inch long at week 8.
You swallow and suck your thumb, usually favoring your right one. You can sense light touch with the palms of your hands and the soles of your feet.
Your fingernails and toenails are growing at 10 weeks. Your fingerprints form, as well.
You are capable of mulitple facial expressions. By week 12, you have your taste buds.
By 13 to 14 weeks, most of you is touch-sensitive.
By this time, your gender distinctions emerge, as female fetuses will begin to move their jaws more.
By 14 weeks, if you are touched near the mouth, you will show the same rooting reflex that newborns use to find their mother’s nipple when breastfeeding.
Between 14 and 18 weeks, your mother will experience an event known as“quickening” as she feels you move for the first time.
Around 19 weeks, your movement, breathing and heart rate fall into daily cycles known as circadian rhythms.
Your cochlea, the organ of hearing reaches adult size within the fully developed inner ear by 20 weeks. Now you hear and respond to a growing mixture of sounds. Your external ear is complete by week 21.
At 20 weeks, your Rapid Eye Movements (REM) are first seen. “After birth, REM are known to occur during the dream stage of sleep.”
Your breathing motions are present 14% of the time.
All of your skin layers and structures, including glands and hair follicles, are present by your 23rd week.
You respond to taste by 25 weeks. Your swallowing increases when sweet substances are placed in the amniotic fluid, but it decreases with bitter substances, which may alter your facial expression.
You undergo a brain growth spurt as your brain weight increases 400-500%, consuming 50% of your energy between 25 weeks and birth!
At 25 weeks, you acquire additional fat, giving you a more plump appearance. These fat deposits store energy and help to maintain your body temperature after you are born.
Your eyelids reopen by 24 weeks. By 25 weeks, your eyes contain rods and cones. They are highly developed. A total of 200 million rods will detect low levels of light as 14 million cones produce focused, color images. By 26 weeks, your eyes form tears. By 27 weeks, your pupils dilate and constrict in response to light – a reflex that will persist throughout your life.
You distinguish high- and low-pitched sounds.
Your sleep patterns become cyclical.
Your breathing motions are present 30% of the time.
By 32 weeks, alveoli, or air “pocket” cells, begin forming in your lungs. They are the site for O₂/CO₂ exchange and will continue to develop until your 8th year, with the greatest increase occurring between your birth and age 2.
By 32 weeks, you have most of the brain’s 100 billion neurons. Your synapseswill fire when each neuron eventually connects with up to 200,000 other neurons, creating an electrical network of immeasurable complexity. While most of your organs are just 5% of their adult size at birth, your brain and eyes are approximately 25% and 75% of their final sizes respectively.
At week 33, you have developed music and voice preferences. You have exhibited taste preferences by week 34.
Your hand grasp is firm at week 35.
During week 36, you gain weight .
Just before you are born, you are drinking up to 25 ounces per day.
Your major untested fetal organs like the lungs, stomach, intestines and kidneys must kick into gear as you are born. Your brain and heart – organs that you have been using for several months – must automatically adapt to this new world.
Unless noted, information is from the print brochure, The Biology of Prenatal Development: The Embryonic and Fetal Periods, by The Endowment for Human Development, Inc. (2005).
 Alexander Tsiaras with text by Berry Werth, From Conception to Birth: A Life Unfolds, Doubleday, New York City, 2002, p. 265.
Alexander Tsiaras shares his amazing medical visualization of human development from conception to birth.