How Does a Fetus Get Oxygen?

A fetus breathes inside its mother by forming an umbilical cord, a lifeline that starts as thin as a hair but grows as thick as an adult finger and up to 2 feet long by the end of pregnancy.

Lifeline Develops

• A baby's lungs start to develop inside the uterus during the first 13 weeks. The fetus grows an umbilical cord attached to the mother's placenta, connected to the uterine lining.

  Emerging from the baby's stomach, the cord contains two arteries and a vein. The vessels form connecting coils, embedded in a jelly with a thin cover, resembling a soft, twisted rope.

  As the mother inhales, oxygen enters her blood through the heart and lungs, and oxygen travels through the placenta and into the fetus's tiny blood vessels. The baby rids its body of carbon dioxide the same way, except in reverse.

Lake of Blood

• Using umbilical cord arteries attached to the placenta, a fetus's heart pumps its own blood. The placenta's finest branches lie bathed in the mother's blood; they are drained by the tributaries of the umbilical vein, which takes it back to the fetus to flow back to the heart. The baby's used blood is pushed through arteries and refreshed blood returns via veins to the heart.

  Meanwhile, the mother's placenta blood forms a lake that bathes the fetus's smallest vessels, or capillaries. Unlike the fetal blood in the placenta kept in vessels, maternal blood flows into 80 to 100 small, twisting uterine arteries that empty into the five-ounce lake. This blood is refreshed three or four times every minute.

Hazards

• In rare cases, the placenta detaches from the uterus and the fetus may get less oxygen, posing a risk to the baby. Only 1 percent of pregnant women experience this, and it usually happens during the last 12 weeks. Women who've already had children, are older than 35, have had detachment before or have sickle cell anemia are at greater risk.

  During labor, the cord may be a hazard; it can move out of place and be compressed by the baby's head pressing against the mother's pelvis, obstructing the blood flow, or it may wrap around the baby's neck. The cord is usually cut with two clamps immediately after birth.

After Childbirth

• Also after birth, the baby's temperature drops, causing the jelly covering the cord to swell and collapse the blood vessels within. This creates a natural clamp and can take only five minutes if left alone.

  When a child starts breathing, the heart and circulation system take over, and the baby uses its lungs to infuse with oxygen. Chemical changes and the cord's stretching and cooling help to squeeze the cord's muscles.

Vein Disappears

• The vein flowing through the liver and connecting the left umbilical vein with the fetus's inferior vena cava, the body's largest vein returning blood to the heart from areas below the diaphragm, loses its circulatory function after birth but remains part of the liver.

  Within a week or two, the baby's cord shrivels and separates from the navel. Mothers are instructed to keep the area clean and talk to their doctor about any abnormal changes.