Genetically, children are a blend of code from both of their parents. But for the first nine months of development, a fetus gets just about everything else from its mother. The two individuals' systems are so intertwined that even after birth, material from a fetus can linger in the mother's body for decades.

The process means major changes for both mother and child. Beginning about three months into pregnancy, the growing fetus and enlarging uterus become visible as a small belly bump. At the end of the second trimester, a woman's uterus has usually reached the size of a papaya to accommodate a 23-centimeter-long fetus, and by the time of a full-term delivery, the uterus will have expanded to about the size of a watermelon, shifting other internal parts around to make room.

In addition to sheer growth, the developing organs and features of the fetus require extra input from the mother's body. In the second two trimesters of pregnancy, women often need an extra 300 calories a day and a range of nutrients to support healthy growth of a fetus—and even more after delivery if the child is being breastfed.

After birth, certain biochemical compounds are at work in both mother and child to sustain and accentuate the bond between the two. Hormones, such as oxytocin, surge in mothers after labor and during breast-feeding, promoting social and emotional bonding with the infant. And in babies, just being touched spurs the release of the same compound, helping them, in turn, bond with their moms and other care takers.

This intensive and intimate journey has been illustrated by the Visual MD, revealing the changes that both mother and child go through during their first year together.

Soon after a zygote attaches to the uterine wall, it begins to develop a simple circulatory system and a primitive neural tube that is the precursor to the nervous system. At this stage, many women still do not yet know they are pregnant.

At five weeks, an embryo has developed a four-chambered heart and begun pumping blood. The beginnings of arms and legs start to appear as small buds—along with a tail, which will eventually transform into the tailbone. Six weeks is often when the mother's body begins to physically react to the new addition, often with nausea or food cravings. 

In the next two weeks, the embryo becomes much more recognizable, complete with fingers, eyelids, ears and even some important parts of the brain (including the cerebral cortex and pituitary gland). Even though the physical details are developing rapidly, the embryo is still only about 1.25 centimeters long.

In the second trimester of pregnancy, the basic systems and organs established during the first 12 weeks continue to develop and differentiate. By week 15, bones have begun to develop. Around 18 weeks, a fetus can begin to hear the maternal heartbeat and even sounds coming from the outside world. And around week 26, the fingernails are apparent.

During pregnancy, a woman's body must ramp up many of its systems to support the growing fetus inside of it. The cardiovascular system has to pump 30 to 50 percent more blood, sending much of the extra blood supply directly to the uterus. The increase in blood and circulation also means more work for the kidneys and lungs.

During the birthing process, the cervix expands to about 10 centimeters to allow the baby's head to pass through the birth canal. The event can be a strenuous one for both mother and baby, and both bodies adjust to accommodate it. Blood output from the mother's heart jumps, and hormone levels shift. Babies are forced to adapt quickly to their new environment:, and they start to breathe with their lungs (rather than getting oxygenated blood from the mother) and regulate their own body temperature.

As mother and baby adjust to the baby being outside of the womb, continued close contact is be important for both. Ample skin-to-skin contact—in addition to breast-feeding—helps ease this transition. Still, each body needs time to recover. It takes about six weeks after delivery for a woman's blood- flow levels and uterus size to return to pre-pregnancy states.

A year to 16 months after conception, the baby is able to sit up unaided. By this time, the muscles to support the large weight of the head have developed. The body's other systems, which had started to emerge in the embryo so early developed, will continue to grow and strengthen as the baby begins to explore the world.