The Fetal Development Timeline

Each of the three trimesters of pregnancy lasts approximately three months (totaling about 40 weeks). In that time, a single cell develops into an embryo, then a fetus and finally a fully developed baby. The mother's uterus creates a placenta and umbilical cord to nourish the fetus, and her internal organs are rearranged as her uterus expands to compensate for the growing fetus.

First Trimester: Embryo to Fetus

The first trimester of pregnancy lasts from conception to the end of week thirteen. An embryo develops after fertilization of the egg (ovum). Initially a single cell, the embryo travels down the fallopian tube. As it moves down the tube, the embryo produces more cells, growing rapidly. A 32-cell embryo is called a morula.

A few days into pregnancy, the tiny embryo reaches the uterus. The embryo embeds itself into the uterine wall and continues to grow rapidly.

As soon as 5 weeks into the pregnancy, ultrasound can detect the embryo. The beginnings of arm and leg buds can be seen.

At 6 weeks, the embryo's heart beats, lungs are forming and webbed hands and feet develop. Changes in the uterus and hormone levels may cause fatigue, nausea and increased urination in the mother.

At 7 weeks of pregnancy, elbows and toes become visible on ultrasound.

After 9 weeks, the embryo begins developing facial features and well-developed fingers and toes. The embryo starts moving, but movement isn't noticed until later in the pregnancy.

Second Trimester: The Fetus Quickens

After 14 weeks , the embryo becomes a fetus; it measures six inches, and has developed a chin and nose. The placenta continues to develop along the uterus. The placenta connects the uterus to the umbilical cord and fetus. Nutrients, waste and other substances pass between mother and fetus through the placenta.

Sometime between 16 and 20 weeks into pregnancy, quickening occurs; these are fetal movements felt by the mother. The fetus can now detect light and hear sounds. The placenta continues to develop along the uterine lining, and the uterus continues to enlarge. Uterine expansion makes the pregnancy visible.

At 20 weeks the size of the f etus size makes the uterus increasingly cramped. The uterus puts pressure on the lungs, bladder, stomach and kidneys. Fetal heartbeat is detectable with a stethoscope.

At 24 weeks, the fetus weighs about two pounds and measures about eleven inches. The fetus has reached the "cusp of viability;" if born now, the fetus has a fifty percent chance of survival.

Third Trimester of Pregnancy

The third trimester of pregnancy begins at week 27. A fetus born now has an 85 percent chance of survival.

Between 30 and 37 weeks , fetal growth continues. The fetus is approximately eighteen inches long by week thirty. Uterine space is now so limited that the umbilical cord often wraps around the fetus, and fetal movement is felt forcefully through the uterine wall.

By 34weeks, the fetal lungs mature. The pregnancy is considered "full-term" between weeks 37 and 42. Induced labor is required if the pregnancy lasts beyond42 weeks. The pressure on the uterus of the still-growing fetus can cause health problems to both mother and fetus if a pregnancy extends past 42 weeks.

Resources

Beers, M. H. & Berkow, R. (ed). Normal pregnancy, labor, and delivery. The Merck Manual of Diagnosis and Therapy, 17th Edition. Merck Research Laboratories, NJ, 1999.

Callahan, T.L., Caughey, A.B., Heffner, L.J. (2001). Blueprints in Obstetrics and Gynecology, 2nd ed. Blackwell Publishing, MA.

Children's Hospital of the King's Daughters. (updated 2004). The first trimester.

Children's Hospital of the King's Daughters. (updated 2004). The second trimester.

Children's Hospital of the King's Daughters. (updated 2004). The third trimester.

Mayo Foundation for Medical Education and Research. (2003). Understanding your body during pregnancy.

National Center on Birth Defects and Developmental Disabilities. (updated 2004). ABC's . . . Pregnancy tips.

Nemours Foundation. (nd). Pregnancy calendar.