Every day is a new process of growth for that developing baby. Some of the most important growth occurs before you even start showing. As your belly expands–and you can see those little hands and feet feeling their way around–your baby adds on weight and height to prepare for a healthy birth.
A zygote is the first developmental stage in a new baby’s life. According to the University of Maryland Medical Center, a zygote is formed when a single sperm combines with a single egg; the resulting fusion contains all of the DNA necessary to develop into a baby. Half of the genetic information comes from the father (through the sperm) and half from the mother (through the egg). After formation, the zygote begins a journey down the fallopian tube. It starts dividing to form a clump of cells.
Once the zygote has divided sufficiently, it forms an inner group of cells protected by an outer layer. This is known as the blastocyst. The inner cells, according to UM Medical Center, will become the embryo; the outer cells will become a protecting and nourishing layer of membranes. During this developmental stage, the blastocyst reaches the uterus and attaches to the uterine lining, which is how it receives nourishment from the mother. Both zygote and blastocyst stages of development are usually complete by 7 days after conception.
The embryo is a stage which lasts through most of the first trimester, according to UM Medical Center. During this time of growth, the baby will begin developing its brain, heart and spinal cord; by week 5 of the pregnancy, the tiny heart will be beating with a regular rhythm. The embryo stage is completed around week 8; by this point, all essential organs have formed or begun to form. The developing baby has arms, legs, hands, feet, and facial features.
The fetal period, according to UM Medical Center, continues from around week 8 through the end of the pregnancy. During this time, the baby develops into what we think of as a baby and grows rapidly. By week 12, the face is well-formed. At this time, genitals are also differentiated, and baby can make a fist with its hand. Between the weeks 13 to 16, the fetus begins moving actively (though it’s not often apparent, especially to first-time mothers, until a bit later). From weeks 17 to 20, the baby develops hearing and moves more. He also grows eyelashes, eyebrows and nails. Weeks 21 through 28 include the beginning of fat storage, rapid brain development, and a more advanced respiratory system. There’s a great increase of body fat from weeks 29 through 32. By the last few weeks of pregnancy, the fetus has fully developed bones and mature lungs.