This pamphlet explains

how pregnancy begins how the fetus grows and develops changes in your body during pregnancy how your due date is calculated

How Pregnancy Begins

Fertilization, the union of an egg and a sperm, is the first step in a complex series of events that leads to pregnancy. Fertilization 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.

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.

How A Baby Grows During Pregnancy

The First Month
½ inch, less than 1 ounce

• 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.

• 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.

• 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.

• 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.

• The sucking reflex develops. If the hand floats to the mouth, the fetus may suck his or her thumb.
• He or she is more active. You may be able to feel him or her move.
• The fetus sleeps and wakes regularly.
• Nails grow to the tips of the fingers.
• 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.

The Sixth Month
12 inches, 1–1½ pounds

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.

The Seventh Month
14 inches, 2–2 1/2 pounds

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.

The Eighth Month
18 inches, 5 pounds

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.

The Ninth Month
20 inches, 6–9 pounds

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 1/2 pound per week this month.

Conversion Chart

1 inch = 2.54 centimeters   1 ounce = 28 grams
1 pound = 453.59 grams

Changes During Pregnancy

Many changes take place inside your body as your baby grows. You cannot see most of these changes at first. If you have concerns or questions about what your body is going through, talk with your health care provider. He or she can offer tips for dealing with pregnancy changes and also assure you that most of what you are feeling is normal and determine what may not be normal.

Your Due Date

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. These last about 12–13 weeks each (or about 3 months):

1st trimester: 0–13 weeks (Months 1–3) 2nd trimester: 14–27 weeks (Months 4–6) 3rd trimester: 28–40 weeks (Months 7–9)

The day your baby is due is called the “estimated date of delivery,” or EDD. Although only about 1 in 20 women give birth on their exact due date, your EDD is useful for a number of reasons. It is used as a guide for checking your baby’s growth and your pregnancy’s progress. The EDD gives a rough idea of when your baby will be born. Most women go into labor within about 2 weeks before or after their due date.

The First Trimester

Your period stops. Your breasts may become larger and more tender. Your nipples may stick out more. You may need to urinate more often. You may feel very tired. You may feel nauseated and even vomit. You may crave certain foods or lose your appetite. You may have heartburn or indigestion. You may be constipated. You may gain or lose a few pounds.

 
The Second Trimester

Your appetite increases and nausea and fatigue may ease. Your abdomen begins to expand. By the end of this trimester, the top of your uterus will be near your rib cage. The skin on your abdomen and breasts stretches and may feel tight and itchy. You may see stretch marks. Your abdomen may ache on one side or the other, as the ligaments that support your uterus are stretched. A dark line, the linea nigra, may appear down the middle of your stomach from your navel to your pubic hair. You may get brown patches (chloasma, or the “mask of pregnancy”) on your face. Your areolas, the darker skin around your nipples, may darken. Your feet and ankles may swell. You may feel your uterus in your lower abdomen.

The Third Trimester

You can feel the baby’s movements strongly. You may be short of breath. You need to urinate more often as the baby drops and puts extra pressure on your bladder. Colostrum—a yellow, watery pre–milk—may leak from your nipples. Your navel may stick out. You may have contractions (abdominal tightening or pain). These can signal false or real labor.

Finally...

Knowing how your baby grows and develops can help you prepare for the coming weeks. The due date helps your health care provider measure the growth of the fetus and the progress of your pregnancy. If you have any questions about what is happening with your baby or your body, talk to your health care provider.

Glossary

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 away waste from the fetus.

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

This Patient Education Pamphlet was developed by the American College of Obstetricians and Gynecologists. Designed as an aid to patients, it sets forth current information and opinions on subjects related to women’s health. The average readability level of the series, based on the Fry formula, is grade 6–8. The Suitability Assessment of Materials (SAM) instrument rates the pamphlets as “superior.” To ensure the information is current and accurate, the pamphlets are reviewed every 18 months. The information in this pamphlet 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 the institution or type of practice, may be appropriate.

Copyright © February 2010 by the American College of Obstetricians and Gynecologists. All rights reserved. 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.

ISSN 1074-8601

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