A new human being forms at fertilization, when a man’s sperm fuses with a woman’s egg. Within the next two decades, that newborn becomes a 30 trillion cell adult! Before a new life can form, a woman’s body must prepare for pregnancy, and a man’s body must produce sperm.

In the third week gestation, the sperm and the egg unite to create a unique single-celled human, the zygote, with all the genetic instructions to form an adult with distinct physical features. The embryo follows a predictable set of cell divisions for the first week after conception, forming a morula and blastocyst as the developing human travels through the mother’s fallopian tubes.

Just as infants need a safe, nourishing environment to grow outside the womb, the embryo needs a safe place to grow for the next 37 weeks. During the fourth week, the preborn baby attaches to his mother in a process called implantation.

Remember how the zygote and the early embryonic cells could become many different types of cells in the human body? In the fifth week, the cells specialize based on their position in the embryo. Brain tissue and heart tissue begin to develop.

In the sixth week, the embryonic heart starts beating rhythmically, moving oxygenated blood throughout the developing embryo. Without the beating heart, the embryo cannot survive. The neural tube, which becomes the brain and spinal cord, forms.

In week seven, the preborn baby is 10,000 times bigger than he was at conception, about the size of a blueberry. Almost all the major organs have started to form, and the brain starts growing at an average rate of 250,000 neurons per minute.

In the eighth week, the embryo starts moving and responds to a light touch. This shows that neurons and muscles form functional connections, the same connections that will underlie complex thought processes in an adult. The hands and the face continue developing this week.

In week nine, networks of neurons produce brain waves that have been recorded as early as 8 ½ weeks gestation. The embryo spontaneously moves his arms, hands, and legs, and may even hiccup. The embryo’s heart now has all four chambers.

By week 10, more than 90% of the body parts have formed. Already, the embryo will show a preference for either her right or left hand as she moves in the womb. The eyeballs finish forming.

In week 11, the fetus moves a lot! In fact, the fetus does not usually stay still for more than 13 minutes at a time! Male and female genitalia start developing.

By week 12, the fetus’s heart has beat over 10 million times! The fingernails start growing and fingerprints start forming. If something tickles the sole of the fetus’s foot, she will curl her toes. Tooth buds are developing as well.

In week 13, the fetal heart pumps about 6 quarts of blood per day. Furthermore, antibodies from the mother transfer to the fetus, so vaccines given to the mother often provide the child with initial protection against some diseases as well.

In week 14, the intestines start absorbing nutrients and the fetus starts making goal-oriented hand movements. Mature tastebuds cover the tongue, and the placenta provides a barrier that protects against most bacteria and some viruses.

Scientific advancements provide clear evidence that the fetus feels pain by 15 weeks gestation, and possibly even earlier. Eyebrows form and the fetus moves each finger separately. Boys and girls move differently in the womb, and may respond to loud noises.

In weeks 17 and 18, the mother may feel her preborn child moving. Also, one research study shows that a fetus as young as 18 weeks responds to a painful procedure with a stress response that includes recoiling and increasing circulating stress hormones in his bloodstream.

In weeks 19 and 20, the fetus shows a clear response to pain in multiple scientific studies, and early brain pathways are mature enough to create a perception of pain. The fetus also practices breathing, crying, and breastfeeding in the womb. The fetal heart has beat over 20 million times and circulates about 55 quarts of blood per day.

The youngest premature baby to survive was born at 21 weeks and 1 day gestation (132 days premature). As hospitals invest in life-saving technologies to save their smallest, most miraculous patients, the age of viability gets younger and younger, bringing joy and hope to many families.

In month six, the fetus consistently responds to loud noises, music, reading, and singing with changes in behavior. The eyes continue maturing and respond to changes in light. The fetus practices breathing in the womb.

At seven months the fetus shows a variety of sleep and wake patterns throughout the day. The fetus overproduces neurons such that at 28 weeks, he has more neurons than he will ever have again. The fetus can start learning sound patterns, and can smell different odors in the amniotic fluid.

In month eight the fetal brain is ready to learn! At 34 weeks, neurons are creating 40,000 new connections, called synapses, every second! With repeated exposures, the fetus can learn flavors, nursery rhymes, songs, and even words that she will recognize after birth!

In month nine, the lungs finish maturing and the baby is born. In the womb, the infant’s lungs are half-filled with amniotic fluid, but multiple processes help clear the lungs. Most babies are born around 40 weeks. A newborn has terrible visual acuity and limited color vision. However, she can identify her mother simply by her smell, voice, or a silent video of her face.