By Joanne Stone, Keith Eddleman, Mary Duenwald

On or about the fifth day of development, the blastocyst attaches to the blood-rich lining of the uterus during a process called implantation. Part of the blastocyst grows to become the embryo (the baby in the first eight weeks of development), and the other part becomes the placenta (the organ that implants into the uterus to provide oxygen and nourishment to the fetus and eliminate its waste products).

From the blastocyst, the embryo develops into three different tissue layers: the endoderm, mesoderm, and ectoderm. These three layers ultimately give rise to all of the structures of the body and are initially organized into a flat disk.

Around the beginning of the fourth week of embryonic development, the flat disk begins to fold and form a cylinder. At this point, the embryo begins to take on the form of the general body plan, with a mouth region and an anal region.

Between weeks four and eight, all of the organ systems that you find in an adult will be forming. After the eighth week of your pregnancy, the developing embryo is referred to as a fetus.

Amazingly, by this time almost all the baby’s major organs and structures are already formed. The remaining 32 weeks allow the fetus’s structures to grow and mature. On the other hand, the brain, although also formed very early, isn’t mature at birth; rather it continues to develop into early childhood.

Your baby grows within the amniotic sac in the uterus. The amniotic sac is full of clear fluid, known as amniotic fluid. This water balloon-like structure actually comprises two thin layers of membrane called the chorion and amnion (which together are known as the membranes).

When people talk about water “breaking,” they’re referring to the rupturing of those membranes that line the uterus’s inner walls. The baby “swims” in this fluid and is attached to the placenta by the umbilical cord. The figure shows a diagram of an early pregnancy, including a developing fetus and the cervix, which is the uterus’s opening. The cervix opens up, or dilates, when you’re in labor.

View of how an embryo grows in a woman's uterus.

Credit: Kathryn Born, MA

The placenta begins to form soon after the embryo implants in the uterus. Maternal and fetal blood vessels lie very close to one another inside the placenta, which allows various substances (such as nutrients, oxygen, and waste) to transfer back and forth. The mother’s blood and the baby’s blood are in close contact, but they don’t actually mix.

The placenta grows like a tree, forming branches that in turn divide into smaller and smaller ones. The tiniest buds of the placenta are called the chorionic villi, and it’s within these villi that small fetal blood vessels form. About three weeks after fertilization, these blood vessels join to form the baby’s circulatory system, and the heart begins to beat.

Menstrual weeks are weeks from the last menstrual period, not weeks from conception. So at eight weeks, the baby is really six weeks from conception.

By the end of the eighth week, arms, legs, fingers, and toes begin to form. In fact, the embryo begins to perform small, spontaneous movements. If you have an ultrasound examination performed in the first trimester, you can see these spontaneous movements on the screen.

The brain enlarges rapidly, and ears and eyes appear. The external genitalia also emerge and can be differentiated as male or female by the end of the 12th week, although sex differences are not yet detectable by ultrasound.

By the end of the 12th week, the fetus is about 4 inches long and weighs about 1 ounce. The head looks large and round, and the eyelids are fused shut. The intestines, which protruded slightly into the umbilical cord at about week 10, are by this time well inside the abdomen.

Fingernails appear, and hair begins to grow on the baby’s head. The kidneys start working during the third month. Between 9 and 12 weeks, the fetus begins to produce urine, which you can see within the small fetal bladder on ultrasound.