Making its longest journey until the big move nine months from now, your developing baby (at this point called a zygote, or fertilized ovum) travels from the fallopian tube and into the uterus (or womb). After fertilization, the zygote begins a process of rapid cell division, and by day four it has formed a small solid cluster of cells known as a morula (after moris, which is Latin for mulberry). The morula finishes the trip down the fallopian tube, reaching the uterus about three to four days after fertilization.

The Blastocyst

By the fifth or sixth day, your baby takes on its third name change in less than a week as the morula grows to a blastocyst. The blastocyst contains two distinct cell layers with a cavity at the center. The inner layer will evolve into the embryo, and the outer layer will develop into the placental membranes — the amnion and the chorion. Within days, the blastocyst nestles into the nutrient-rich lining of your uterus (the endometrium) as implantation begins about one week after conception. Wispy fingers of tissue from the chorion layer called chorionic villi will anchor the blastocyst firmly to your uterine wall, where they will begin to build a network of blood vessels. These villi are the start of the placenta, a spongy oval-shaped structure that will feed the fetus (via the umbilical cord) with maternal nutrients and oxygen throughout pregnancy.

From conception to blastocyst

The Embryo

About fifteen days after conception, the blastocyst officially becomes an embryo. Next to the embryo floats the yolk sac, a cluster of blood vessels that provide blood for the embryo at this early stage until the placenta takes over. The embryo is surrounded by a watertight sac called the amnion (or amniotic sac). The amniotic fluid that fills the sac provides a warm and weightless environment for your developing baby. It also serves as a sort of embryo airbag (or in this case, fluid bag), protecting baby from the bumps and bustle of your daily routine. Nature efficiently double bags your baby, surrounding the embryo and the amnion with a second membrane called the chorion.

During this first four weeks of development, your embryo has laid the groundwork for most of its major organ systems. As month one draws to a close, baby's heart is beating (although you won't be able to hear it for several weeks yet), lung buds have appeared, and construction of the gastrointestinal system and liver are well underway. The neural tube, the basis of the baby's central nervous system, has developed, and the forebrain, midbrain, and hindbrain are defined. The embryo is starting to look more like a person, too. The first layer of skin has appeared, facial features are surfacing, and arm and leg buds — complete with the beginnings of both feet and hands — are visible. It's an amazing list of accomplishments considering your baby is about the size of a raisin (less than a quarter of an inch long).

Now an embryo, your baby at one month of gestation.