How Pregnancy Works

First Trimester: From Embryo to Fetus

The embryo at six weeks
The embryo at six weeks
Image courtesy A.D.A.M.

Even though the embryo is still the size of a sesame seed, the mother-to-be will probably start feeling the first twinges of pregnancy. Morning sickness, frequent urination, sleepiness, and food cravings or aversions are all common. Her breasts may swell and become tender. Some women start to put on weight, but others actually lose weight from morning sickness. At this point in the pregnancy, the woman will have her first prenatal Ob/Gyn visit. By the fifth week, an ultrasound may be able to pick up a heartbeat.

The mother needs to be especially careful during this first trimester, during the formation of the delicate organs. Pregnant women should avoid alcohol, certain medications, caffeine, and smoking. They should also continue to take prenatal vitamins containing folic acid, eat a healthy diet and exercise regularly.

Between 9 and 12 weeks, women over age 35 and those who have a family history of chromosomal abnormalities will probably have a chorionic villus sampling (CVS). This test detects Down syndrome and other chromosomal abnormalities. Using an ultrasound to guide him, the doctor will remove a small piece of placental tissue and test it for these conditions.

In the last few weeks of the first trimester, the embryo really starts to take shape. The facial structures begin to form and become recognizable. The neural tube, which will form the brain and spinal cord, develops. Little buds emerge and grow into arms and legs.

Around week 8, the embryo becomes a fetus. The kidneys, liver, brain, and lungs are all beginning to function. The fingers and toes are separate and the external genitalia are formed. At 12 weeks, the fetus is about three inches long and weighs about one ounce. At the end of the first trimester, many pregnant women find that their clothes are getting tight, although they may not yet be ready for maternity clothes.

In the next section, we'll discuss what happens during the second trimester.