FAQ Pregnancy
FAQ156, August 2011

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How Your Baby Grows During Pregnancy

How does pregnancy begin?

Fertilization, the union of an egg and a sperm, is the first step in a complex series of events that leads to pregnancy. Fer-tilization takes place in the fallopian tube. During the next few days, the fused egg and sperm move through the fallopian tube to the lining of the uterus. There it implants and starts to grow.

The cluster of cells that reaches the uterus will become the fetus and the placenta. The placenta functions as a life-support system during pregnancy. It delivers oxygen, nutrients, and hormones from mother to fetus.

How long does a normal pregnancy last?

A normal pregnancy lasts about 280 days (about 40 weeks), counting from the first day of your last menstrual period. A normal range, however, is from as few as 259 days to as many as 294 days (37–42 weeks). The 40 weeks of pregnancy are divided into three trimesters. Each trimester lasts about 12–13 weeks each (or about 3 months).

How does the uterus change during pregnancy?

During pregnancy, the lining of a woman’s uterus thickens and its blood vessels enlarge to nourish the fetus. As pregnancy progresses, the uterus expands to make room for the growing baby. By the time your baby is born, your uterus will be many times its normal size.

What happens during the first month of pregnancy?

During the first month of pregnancy, the following occurs:

  • The fertilized egg attaches to the lining of the uterus. Some of these cells will grow into a baby. Other cells will form the placenta.
  • Arms and legs begin to form.
  • The brain and spinal cord begin to form.
  • The heart and lungs begin to develop. The heart begins to beat near the end of this month.

What happens during the second month of pregnancy?

During the second month of pregnancy, the following occurs:

  • Eyelids form, but remain closed.
  • The inner ear begins to develop.
  • Bones appear.
  • Ankles, wrists, fingers, and toes form.
  • The genitals begin to develop.
  • By the end of the month, all major organs and body systems have begun to develop.

What happens during the third month of pregnancy?

During the third month of pregnancy, the following occurs:

  • Twenty buds for future teeth appear.
  • All internal parts are formed, but are not fully developed.
  • Fingers and toes continue to grow. Soft nails begin to form.
  • Bones and muscles begin to grow.
  • The intestines begin to form.
  • The backbone is soft and can flex.
  • The skin is almost transparent.
  • The hands are more developed than the feet.
  • The arms are longer than the legs.

What happens during the fourth month of pregnancy?

During the fourth month of pregnancy, the following occurs:

  • Eyebrows, eyelashes, and fingernails form.
  • Arms and legs can flex.
  • External sex organs are formed.
  • The skin is wrinkled and the body is covered with a waxy coating (vernix) and fine hair (lanugo).
  • The placenta is fully formed.
  • The outer ear begins to develop.
  • The fetus can swallow and hear.
  • The neck is formed.
  • Kidneys are functioning and begin to produce urine.

What happens during the fifth month of pregnancy?

During the fifth month of pregnancy, the following occurs:

  • The sucking reflex develops. If the hand floats to the mouth, the fetus may suck its thumb.
  • The fetus is more active. You may be able to feel movement.
  • The fetus sleeps and wakes regularly.
  • Nails grow to the tips of the fingers.
  • The gallbladder begins producing bile, which is needed to digest nutrients.
  • In girls, the eggs have formed in the ovaries.
  • In boys, the testicles begin to descend from the abdomen into the scrotum.

What happens during the sixth month of pregnancy?

During the sixth month of pregnancy, the following occurs:

  • Real hair begins to grow.
  • The brain is rapidly developing.
  • The eyes begin to open.
  • Finger and toe prints can be seen.
  • The lungs are fully formed, but not yet functioning.

What happens during the seventh month of pregnancy?

During the seventh month of pregnancy, the following occurs:

  • The eyes can open and close and sense changes in light.
  • Lanugo begins to disappear.
  • The fetus kicks and stretches.
  • The fetus can make grasping motions and responds to sound.

What happens during the eighth month of pregnancy?

During the eighth month of pregnancy, the following occurs:

  • With its major development finished, the fetus gains weight very quickly.
  • Bones harden, but the skull remains soft and flexible for delivery.
  • The different regions of the brain are forming.
  • Taste buds develop and the fetus can taste sweet and sour.
  • The fetus may now hiccup.

What happens during the ninth month of pregnancy?

During the ninth month of pregnancy, the following occurs:

  • The fetus usually turns into a head-down position for birth.
  • The skin is less wrinkled.
  • The lungs mature and are ready to function on their own.
  • Sleeping patterns develop.
  • The fetus will gain about ½ pound per week this month.


Egg: The female reproductive cell produced in and released from the ovaries; also called the ovum.

Fertilization: Joining of the egg and sperm.

Fetus: The developing offspring in the uterus from the ninth week of pregnancy until the end of pregnancy.

Hormones: Substances produced by the body to control certain functions.

Placenta: Tissue that provides nourishment to and takes waste away from the fetus.

Sperm: A male cell that is produced in the testes and can fertilize a female egg cell.

If you have further questions, contact your obstetrician–gynecologist.

FAQ156: Designed as an aid to patients, this document sets forth current information and opinions related to women’s health. The information does not dictate an exclusive course of treatment or procedure to be followed and should not be construed as excluding other acceptable methods of practice. Variations, taking into account the needs of the individual patient, resources, and limitations unique to institution or type of practice, may be appropriate.

Copyright August 2011 by the American College of Obstetricians and Gynecologists. No part of this publication may be reproduced, stored in a retrieval system, posted on the Internet, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher.