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Uterine changes pregnancy


Changes During Pregnancy | ACOG

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Month 1 to 2
Weeks 1 to 8
[Image of an embryo in a uterus during the first or second month of pregnancy]

  • The egg is fertilized by sperm and a growing ball of cells called the blastocyst implants in the uterus.

  • Week 5 begins the embryo stage of development.

  • The brain and spine begin to form, followed by the neural tube.

  • Cardiac tissue starts to develop.

  • Parts of the face take shape and the inner ear begins to develop.

  • Arm and leg buds appear, and then webbed fingers and toes emerge.

  • The long tube that will become the digestive tract takes shape.

By the end of week 8, the embryo is about half an inch long.

Month 3
Weeks 9 to 12
[Image of a fetus in a uterus during the third month of pregnancy]

  • Cartilage for the limbs, hands, and feet is forming but won’t harden into bones for a few weeks.

  • Eyelids form but remain closed.

  • The head develops a rounded shape.

  • Week 11 begins the fetus stage of development.

  • The fetus makes breathing-like movements and swallows amniotic fluid.

  • The kidneys are making urine, the pancreas is making insulin, and fingernails have formed.

By the end of week 12, the fetus is about 2 inches long and weighs about half an ounce.

Month 4
Weeks 13 to 16
[Image of a fetus in a uterus during the fourth month of pregnancy]

  • By week 13, all major organs have formed and will continue to develop.

  • Bones are hardening, especially the long bones.

  • The skin is thin and see-through but will start to thicken soon.

  • At week 14, the neck is defined, and the lower limbs are developed.

  • The fetus’s hearing begins to develop.

  • The lungs begin to form tissue that will allow them to exchange oxygen and carbon dioxide after birth.

  • Limb movements become more coordinated.

By the end of week 16, the fetus is more than 4 inches long and weighs more than 3 ounces.

Month 5
Weeks 17 to 20
[Image of a fetus in a uterus during the fifth month of pregnancy]

  • The fetus is more active now, and cardiac activity may be seen on an ultrasound exam.

  • At week 18, the fetus can hear sounds.

  • The part of the brain that controls motor movements is fully formed.

  • The digestive system is working.

  • At week 19, the ears, nose, and lips may be recognizable on an ultrasound exam.

  • Soft, downy hair called lanugo is starting to form all over the body.

By the end of week 20, the fetus is more than 6 inches long and weighs less than 11 ounces.

Month 6
Weeks 21 to 24
[Image of a fetus in a uterus during the sixth month of pregnancy]

  • The fetus’s kicks and turns are stronger now.

  • If the hand floats to the mouth, the fetus may suck its thumb.

  • Eyebrows are visible.

  • At week 23, most of the fetus’s sleep time is spent in rapid eye movement (REM) sleep.

  • Ridges are forming in the hands and feet that later will be fingerprints and footprints.

  • The lungs continue to develop.

By the end of week 24, the fetus is about 12 inches long and weighs about 1½ pounds.

Month 7
Weeks 25 to 28
[Image of a fetus in a uterus during the seventh month of pregnancy]

  • The fetus can respond with movement to familiar sounds, such as your voice.

  • The lungs are now fully formed but not yet ready to function outside the uterus.

  • Loud sounds may make the fetus respond by pulling in arms and legs.

  • The lungs begin making surfactant, a substance needed for breathing after birth.

  • At 27 weeks, more fat is being added to keep the fetus warm.

  • A greasy material called vernix has started to develop. Vernix acts as a waterproof barrier that protects the skin.

By the end of week 28, the fetus is nearly 15 inches long and weighs about 2½ pounds.

Month 8
Weeks 29 to 32
[Image of a fetus in a uterus during the eighth month of pregnancy]

  • The fetus can stretch, kick, and make grasping motions.

  • The eyes can open and close and sense changes in light.

  • The bone marrow is forming red blood cells.

  • At week 31, major development is finished, and the fetus is gaining weight very quickly.

  • In boys, the testicles have begun to descend into the scrotum.

  • At week 32, the fine hair that covered the fetus's body (lanugo) begins to disappear.

By the end of week 32, the fetus is almost 17 inches long and weighs a little more than 4 pounds.

Month 9
Weeks 33 to 36
[Image of a fetus in a uterus during the ninth month of pregnancy]

  • The brain is growing and developing rapidly.

  • The bones harden, but the skull remains soft and flexible.

  • More fat is forming under the skin.

  • The fingernails have grown to the ends of the fingers.

  • During week 36 or 37, most fetuses turn to a head-down position for birth.

By the end of week 36, the fetus is about 18 inches long and weighs a little more than 6 pounds.

Month 10
Weeks 37 to 40
[Image of a fetus in a uterus during the tenth month of pregnancy]

  • The lungs, brain, and nervous system continue to develop.

  • The circulatory system is complete, and so is the musculoskeletal system.

  • The fetus is taking up a lot of space in the amniotic sac and you should continue to feel movement.

  • By now, the fetus’s head may have dropped lower into position in your pelvis.

By the end of week 40, the fetus is 20 inches long and may weigh 7½ to 8 pounds.

 

How the Uterus Grows During Pregnancy

The size of your uterus can help show how long you have been pregnant. The uterus fits inside the pelvis until week 12. By week 36, the top of the uterus is under your rib cage.

[A pregnant woman is shown with dotted lines on her abdomen to mark how the uterus grows during pregnancy. Each line marks four weeks of pregnancy, from week 12 to week 40. The lines move from the bottom of her abdomen to the top of her abdomen.]

 

Changes In Your Body

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 may vomit.

  • You may crave certain foods or lose your appetite.

  • You may have heartburn or indigestion.

  • You may feel bloated and have excess gas.

  • 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.

  • You will begin to feel the fetus move.

  • The skin on your abdomen 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.

  • You may get brown patches, called the “mask of pregnancy,” on your face.

  • Your areolas, the darker skin around your nipples, may darken.

  • Your feet and ankles may swell.

The Third Trimester

  • You can feel the fetus's movements strongly.

  • You may be short of breath.

  • You may need to urinate more often as the fetus drops and puts extra pressure on your bladder.

  • Colostrum—a yellow, watery premilk—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.

PFSI026: This information is designed as an educational aid to patients and sets forth current information and opinions related to women’s health. It is not intended as a statement of the standard of care, nor does it comprise all proper treatments or methods of care. It is not a substitute for a treating clinician’s independent professional judgment. For ACOG’s complete disclaimer, visit www.acog.org/WomensHealth-Disclaimer.

Copyright October 2020 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.

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Anatomy of pregnancy and birth - uterus

Anatomy of pregnancy and birth - uterus | Pregnancy Birth and Baby beginning of content

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What does the uterus look like?

One of the most recognised changes in a pregnant woman’s body is the appearance of the ‘baby bump’, which forms to accommodate the baby growing in the uterus. The primary function of the uterus during pregnancy is to house and nurture your growing baby, so it is important to understand its structure and function, and what changes you can expect the uterus to undergo during pregnancy.

The uterus (also known as the ‘womb’) has a thick muscular wall and is pear shaped. It is made up of the fundus (at the top of the uterus), the main body (called the corpus), and the cervix (the lower part of the uterus ). Ligaments – which are tough, flexible tissue – hold it in position in the middle of the pelvis, behind the bladder, and in front of the rectum.

The uterus wall is made up of 3 layers. The inside is a thin layer called the endometrium, which responds to hormones – the shedding of this layer causes menstrual bleeding. The middle layer is a muscular wall. The outside layer of the uterus is a thin layer of cells.

Illustration showing the female reproductive system.

The size of a non-pregnant woman's uterus can vary. In a woman who has never been pregnant, the average length of the uterus is about 7 centimetres. This increases in size to approximately 9 centimetres in a woman who is not pregnant but has been pregnant before. The size and shape of the uterus can change with the number of pregnancies and with age.

How does the uterus change during pregnancy?

During pregnancy, as the baby grows, the size of a woman’s uterus will dramatically increase. One measure to estimate growth is the fundal height, the distance from the pubic bone to the top of the uterus. Your doctor (GP) or obstetrician or midwife will measure your fundal height at each antenatal visit from 24 weeks onwards. If there are concerns about your baby’s growth, your doctor or midwife may recommend using regular ultrasound to monitor the baby.

Fundal height can vary from person to person, and many factors can affect the size of a pregnant woman’s uterus. For instance, the fundal height may be different in women who are carrying more than one baby, who are overweight or obese, or who have certain medical conditions. A full bladder will also affect fundal height measurement, so it’s important to empty your bladder before each measurement. A smaller than expected fundal height could be a sign that the baby is growing slowly or that there is too little amniotic fluid. If so, this will be monitored carefully by your doctor. In contrast, a larger than expected fundal height could mean that the baby is larger than average and this may also need monitoring.

As the uterus grows, it can put pressure on the other organs of the pregnant woman's body. For instance, the uterus can press on the nearby bladder, increasing the need to urinate.

How does the uterus prepare for labour and birth?

Braxton Hicks contractions, also known as 'false labour' or 'practice contractions', prepare your uterus for the birth and may start as early as mid-way through your pregnancy, and continuing right through to the birth. Braxton Hicks contractions tend to be irregular and while they are not generally painful, they can be uncomfortable and get progressively stronger through the pregnancy.

During true labour, the muscles of the uterus contract to help your baby move down into the birth canal. Labour contractions start like a wave and build in intensity, moving from the top of the uterus right down to the cervix. Your uterus will feel tight during the contraction, but between contractions, the pain will ease off and allow you to rest before the next one builds. Unlike Braxton Hicks, labour contractions become stronger, more regular and more frequent in the lead up to the birth.

How does the uterus change after birth?

After the baby is born, the uterus will contract again to allow the placenta, which feeds the baby during pregnancy, to leave the woman’s body. This is sometimes called the ‘after birth’. These contractions are milder than the contractions felt during labour. Once the placenta is delivered, the uterus remains contracted to help prevent heavy bleeding known as ‘postpartum haemorrhage‘.

The uterus will also continue to have contractions after the birth is completed, particularly during breastfeeding. This contracting and tightening of the uterus will feel a little like period cramps and is also known as 'afterbirth pains'.

Read more here about the first few days after giving birth.

Sources:
The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (Labour and birth), StatPearls Publishing (Anatomy, Abdomen and Pelvis), Department of Health (Clinical practice guidelines: Pregnancy care), Better Health Channel Victoria (Pregnancy stages and changes), Mater Mother's Hospital (Labour and birth information), Royal Australian and New Zealand College of Obstetricians and Gynaecologists (The First Few Weeks Following Birth), Queensland Health (Queensland Clinical Guidelines – maternity and neonatal), King Edward Memorial Hospital (Fundal height: Measuring with a tape measure), Royal Hospital for Women (Fetal growth assessment (clinical) in pregnancy), MSD Manual (Female internal genital organs)

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: October 2020


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