Your Baby in the First Trimester of Pregnancy

The first trimester is the most important time for your fetus. This is the period when all of the major body systems are formed.

  • In the first week after conception, the fertilized egg begins a period of cell division. By the fourth week, the buds of the arms and legs appear, and the nervous system, brain, urinary system, skin, bone, lungs, eyes, ears, and nose begin to form. The heart also begins to beat. Around this time, the embryo (the baby is not called a fetus until around eight weeks) is no larger than a small grain of rice, and it looks like a tadpole.
  • At about the sixth week of pregnancy, the embryo floats in the amniotic sac, a structure filled with salty fluid. At the end of the second month (eight weeks), the placenta - the organ through which food and nutrients (from the mother) flow and waste products (from the baby) leave - is beginning to develop. The placenta is connected to the baby by the umbilical cord. The major organs, the heart, the liver, and the kidneys (as well as every major body system that will appear in the full term newborn) are formed but not fully developed. Bones are growing. At eight weeks, the fetus is about one inch long from head to buttocks.
  • By the 14th week, the sex of the baby is distinguishable and she can move smile, frown, suck, and swallow.
  • During the third month, the placenta is developed and functioning. You'll be able to hear the baby's heartbeat through the use of a Doptone, an instrument that magnifies the sound. The baby now weighs about one ounce, and by the end of the month it will have grown to about three to four inches long. Its face is forming and its fingers and toes have the beginning of nails.

Continue to "Your First Prenatal Visit" journey to parenthood

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Childbirth Connection is a national not-for-profit organization founded in 1918 as Maternity Center Association. Our mission is to improve the quality of maternity care through research, education, advocacy and policy. Childbirth Connection promotes safe, effective and satisfying evidence-based maternity care and is a voice for the needs and interests of childbearing families.
Most recent page update: 9/9/2010