It's around the fourth week of gestation when a pregnancy test typically confirms your suspicions. While you have may have been wondering whether or not you are pregnant, your uterus has been busy at work creating a safe and nurturing environment for your fetus. The early weeks of pregnancy are regarded as a critical time of development. A fetus at 4 weeks is well on its way to developing into a strong, healthy newborn some 36 weeks down the road.

Embryonic Stage

According to BabyCenter.com, week four marks the beginning of the embryonic period. Over the next six weeks, all of your baby's organs will begin to develop and some will even start to function. A fetus at 4 weeks is at its most vulnerable to anything that might interfere with its development.

Size

A 4-week-old embryo is about the size of a small grain of rice or a poppy seed. The embryo consists of two layers known as the epiblast and the hypoblast. These are the launching pads for the future development of a fetus’ organs and body parts. At 4 weeks, a fetus is only about .014 to .04 inches long.

Appearance

An embryo at 4 weeks will begin to bear a resemblance to a small tadpole with eyes, rather than an egg. By the end of the fourth week, its arm and leg buds usually appear.

Gender

Although you can’t tell by looking just yet, it’s already been established whether your fetus will be a boy or a girl. Since sperm carries either an "X" (girl) chromosome or a "Y" (boy) chromosome, gender is determined at the moment of fertilization.

Placenta

During week four, the cells of the primitive placenta are channeling into the lining of the uterus. This will give the placenta an adequate blood supply to provide oxygen and nutrients to the developing fetus.

Looking Ahead

By the end of week four, blood will be pumping and all four heart chambers will be functioning. As your fetus enters week five of gestation, the heart, central nervous system, bones and muscles will start to form. There may even be a hint of skeletal development.