How a Baby Grows During Pregnancy

Pregnancy begins when the male's sperm enters the female's egg. This happens in the fallopian tube and is called fertilization. The fertilized egg is called an embryo until it reaches 9 weeks from the time of fertilization. From 9 weeks until birth it is called a fetus. The fertilized egg moves down the tube into the uterus and attaches to the inside lining of the uterus.

The pregnant woman is responsible for the growth of the embryo/fetus by supplying nourishment and oxygen through the blood stream and placenta to the developing fetus. The uterus becomes larger and pops out from the abdomen more and more as the fetus develops and grows.

A normal pregnancy lasts 280 days, with a range of 259 to 294 days, or 40 weeks. The pregnancy is divided up into three trimesters:

  • First trimester - 0 to 13 weeks.

  • Second trimester - 14 to 27 weeks.

  • Third trimester - 28 to 40 weeks.

The day your baby is supposed to be born is called estimated date of confinement (EDC) or estimated date of delivery (EDD).


  1. First Month: The fertilized egg attaches to the inside of the uterus and certain cells will form the placenta and others will develop into the fetus. The arms, legs, brain, spinal cord, lungs, and heart begin to develop. At the end of the first month the heart begins to beat. The embryo weighs less than an ounce and is ½ inch long.

  2. Second Month: The bones can be seen, the inner ear, eye lids, hands and feet form and genitals develop. By the end of 8 weeks, all of the major organs are developing. The fetus now weighs less than an ounce and is one inch (2.54 cm) long.

  3. Third Month: Teeth buds appear, all the internal organs are forming, bones and muscles begin to grow, the spine can flex and the skin is transparent. Finger and toe nails begin to form, the hands develop faster than the feet and the arms are longer than the legs at this point. The fetus weighs a little more than an ounce (0.03 kg) and is 3½ inches (8.89cm) long.

  4. Fourth Month: The placenta is completely formed. The external sex organs, neck, outer ear, eyebrows, eyelids and fingernails are formed. The fetus can hear, swallow, flex its arms and legs and the kidney begins to produce urine. The skin is covered with a white waxy coating (vernix) and very thin hair (lanugo) is present. The fetus weighs 5 ounces (0.14kg) and is 6 to 7 inches (16.51cm) long.

  5. Fifth Month: The fetus moves around more and can be felt for the first time (called quickening), sleeps and wakes up at times, may begin to suck its finger and the nails grow to the end of the fingers. The gallbladder is now functioning and helps to digest the nutrients, eggs are formed in the female and the testicles begin to drop down from the abdomen to the scrotum in the male. The fetus weighs ½ to 1 pound (0.45kg) and is 10 inches (25.4cm) long.

  6. Sixth Month: The lungs are formed but the fetus does not breath yet. The eyes open, the brain develops more quickly at this time, one can detect finger and toe prints and thicker hair grows. The fetus weighs 1 to 1½ pounds (0.68kg) and is 12 inches (30.48cm) long.

  7. Seventh Month: The fetus can hear and respond to sounds, kicks and stretches and can sense changes in light. The fetus weighs 2 to 2½ pounds (1.13kg) and is 14 inches (35.56cm) long.

  8. Eight Month: All organs and body systems are fully developed and functioning. The bones get harder, taste buds develop and can taste sweet and sour flavors and the fetus may hiccup now. Different parts of the brain are developing and the skull remains soft for the brain to grow. The fetus weighs 5 pounds (2.27kg) and is 18 inches (45.75cm) long.

  9. Ninth Month: The fetus gains about a half a pound a week, the lungs are fully developed, patterns of sleep develop and the head moves down into the bottom of the uterus called vertex. If the buttocks moves into the bottom of the uterus, it is called a breech. The fetus weighs 6 to 9 pounds (2.72 to 4.08kg) and is 20 inches (50.8cm) long.

You should be informed about your pregnancy, yourself and how the baby is developing as much as possible. Being informed helps you to enjoy this experience. It also gives you the sense to feel if something is not going right and when to ask questions. Talk to your caregiver when you have questions about your baby or your own body.