Uterus in Pregnancy - Functions, Position, Size & more

Uterus during Pregnancy

The uterus is a pear-shaped, dynamic female reproductive organ located in the pelvis between the urinary bladder and the rectum. The average dimensions are approximately 8 cm length, 5 cm breadth, and 4 cm thickness, with an average volume between 80 and 200 ml. Together with other vital organs of the reproductive system, the uterus plays an important role in reproduction, menses, implantation of the zygote, gestation, labour, and delivery of the baby. It is under the influence of the hormonal milieu within the body adapting to the different stages of a woman’s reproductive life.

What is a Uterus and How Does it Work?

The uterus is divided into 3 main parts: fundus, body, and cervix. To understand this, imagine a pear upside down. The upper globular, thick part is the fundus, the lower narrow slightly tubular part is the cervix and in between is the body. The uterus has two arm like extensions one on each side at the junction of the fundus and the body called the fallopian tubes.

The ovary is a specialised structure that produces the ovum or the egg. Your ovary starts producing ova even before birth but the maturation and release of these ova begins after the onset of puberty or menses. Each month, either of the ovaries releases one ovum which is picked up by the fimbriae and travels through the fallopian tube to reach the uterine cavity or the inside of the womb. Then it meets the sperm (if you have had unprotected sex).

Where does the egg meet the sperm? The millions of sperms ejaculated into the vagina during unprotected sexual intercourse travel through the cervix, up the body, and meet the egg in the fallopian tube, where one lucky sperm gets to enter the egg to fertilise it. This results in formation of the zygote, which will then develop into a foetus.

How the Uterus Works?

It is quite interesting to see how the uterus functions. The uterus is hollow inside and has a wall that is 3 layers thick. The outermost layer is a very thin layer which forms a coat or an envelope. The middle layer is a thick layer of muscles which forms the main bulk. It gives the strength to the wall of the uterus and is capable of expanding to accommodate a growing baby and contracting to push the baby out during labour.

The inner thin glandular lining is called the endometrium. It is the most active layer responding to all the hormonal changes and is highly specialised. It is formed every month and it prepares itself for conception and pregnancy, waiting for a fertilised egg to arrive and implant itself on it to begin the process of making a beautiful baby. For most of the months after puberty however, what it receives is an unfertilised egg. During such a situation, the inner lining is shed with some blood and that’s what is called menses and this whole cycle is called menstrual cycle

When a fertilised egg does arrive, it gets implanted on the endometrium. Now, it will grow to form a placenta and an embryo. The placenta forms connections with the uterine blood vessels to provide nutrition to the embryo via an umbilical cord. While this is happening the uterus sends signals to the brain to modify the hormone release such that any further egg releases (i.e. ovulation) are stopped and this temporarily ceases the menstrual cycle, thus confirming your pregnancy.

The uterus has a rich network of blood vessels and nerves. The nerves are responsible for the pain due to the contraction of the muscular middle layer during menstruation and during labour.

Function of Uterus in Pregnancy

Pregnancy is a complex process requiring a good niche. There is interplay of various hormones and chemical mediators released by the brain and the uterus that need to be in perfect sync to conceive and maintain the pregnancy. These factors include:

  • Space and support

In pregnancy the uterus provides space and a suitable environment called the amniotic sac (a water bag filled with amniotic fluid) for the foetus to grow comfortably.

  • Connects the mother and the baby

The uterus forms a connection between the mother and the baby via the placenta and umbilical cord to not only provide the necessary nutrition and oxygen but to also remove the waste products and purify the foetal blood until the foetal organs take over.

  • Feedback to the brain

Through the feedback given by the uterus, the brain regulates the hormones throughout the pregnancy to keep the uterus in a relaxed state. At full term, the uterus sends a feedback message to the brain that the baby is ready for the most awaited moment and that is when the hormones change and the uterus begins to contract at the onset of the labour.

  • Labour

Labour depends entirely on the efficient contraction and retraction of the uterine muscle fibres. Gradually the baby is pushed down with a progressive increase in the frequency and the strength of contractions.

  • Haemostasis

On completion of birth the uterus contracts and becomes a hard small ball. It is structured to reduce bleeding and revert to the non-pregnant state.

Changes which Happen to Your Uterus During Pregnancy

Nature beautifully takes care of all the adaptations that are required in the mother’s body for a healthy pregnancy. The changes are in the structure (anatomical changes) and the function (physiological changes) not only in the reproductive organs but all other systems of the body. It is the maternal adaptation to the increasing demands of the growing foetus.

1. Changes in position

The uterus undergoes immense changes during pregnancy, having a significant impact on the body as a whole, altering other organ systems too. Many of these changes are necessary to maintain pregnancy while others are merely side-effects. These changes may cause discomfort to the mother.

Normal anteverted position (forward bending of the uterus on itself) is exaggerated up to 8 weeks. The enlarged uterus lies on the bladder making it incapable of filling to its usual capacity and hence the increased frequency of micturition during early pregnancy is experienced. Afterwards it becomes erect, and near term it is held straight up against the spine by the good tone of the abdominal muscles.

2. Size of uterus during pregnancy

There is a gradual and steady increase in the size of the uterus from the normal 7 x 5 x 3 cm to a bigger 35 x 25 x 22 cm. The increase in size is about 5-6 times.

Size during first trimester

During the first trimester, the uterus is around the size of a grapefruit and starts to grow out of your pelvis even though it is still inside it. This is generally around the 12 weeks stage.

Size during second trimester

In the second trimester, the uterus grows to about the size of a papaya and no longer fits inside               the  pelvis and is somewhere between the navel and breasts.

Size during third trimester

Around the third trimester, the uterus is the shape of a watermelon and will extend from the rib cage to the pubic area.

Size after pregnancy

After pregnancy, the uterus will go back to its original shape inside the pelvis and this process is called involution and takes around 6 weeks to complete.

3. Measuring Uterus

Uterus cannot be measured up until 12 weeks of pregnancy as it is positioned within the pelvic cavity. Beyond 12 weeks it begins to be an abdominal organ when the fundus of the uterus can be palpated. There will be an upshift in the fundal height of the uterus during pregnancy week by week. This is useful to monitor the foetal growth and the amniotic fluid volume. Roughly the distance between the pubic bone and the fundus in centimetres correlates with the week of gestation up to 34 weeks. The examination is done in a lying down position with both the legs folded in the knees and relaxed abdomen.

The examiner tries to feel for the fundus with the sides of the palm. As a rough guide, at 12-14 weeks the fundus should be felt just above the pubic bone. At 2022 weeks it will be felt at the naval. And at about 34-36 weeks it should reach the upper part of abdomen or the epigastric region. If the uterus is not at the desired height at that specific week, it means that it is either small for the age baby, or the volume of amniotic fluid is low.

Other way of measuring with more accuracy is the ultra sonography method that determines the size using sound waves.

4. Changes Inside the Uterus During Pregnancy

The growing size of the uterus is because of the foetus as well as increase in the actual tissue content and blood vessels in the uterus. Accordingly, the uterus dimensions will change as follows:

– Weight: 20-fold increase in the weight of the uterus (50 → 1000 grams)

– Volume: 1000-fold increase (4 → 4000 ml)

– Location: Uterus which is a strictly a pelvic organ until 12 weeks starts to become abdominal

– Blood flow: 10-fold increase (50 → 500 ml/min)

– Shape: Its shape changes from elongated to oval by the 2nd month, to round by 12 weeks, then back through oval to elongated at term.

Problems with Uterus During Pregnancy

There is a wide spectrum of abnormalities of the uterus that can affect pregnancy outcome, with mere variation of normal at one end of the spectrum and gross abnormalities affecting the functioning of the uterus at the other end. Of these some are congenital defects while some uterine problems may be acquired.

Abnormalities of Uterus in Pregnancy

Some common congenital uterine malformations include:

Name Frequency of Occurrence reported among women surveyed Condition
Unicornuate uterus 5% Here, the uterus is half formed or half-sized, making it extremely hard for a pregnancy to occur
Uterine didelphyis 11% Malformation where fusion of the Mullerian ducts do not occur. This results in double crevices, making it hard for pregnancy.
Bicornuate uterus 39% Most common form of abnormality where there is a double uterus with a single vagina or cervix
Septate uterus 34% Partial or incomplete longitudinal uterine cavity septum
Arcuate 7% Slight deviation from a normally developed uterus
Hypoplastic 4% Small and improperly formed uterus, and in some cases, no uterus at all.

Other common acquired uterine abnormalities:

1. Cervical incompetence

In this case the cervical opening or the orifices are unable to stay closed. As a result, it can lead to miscarriage during early pregnancy or preterm delivery if near the delivery date. You will experience vaginal bleeding or premature contractions.

It is treated by cervical en cerclage; as the name suggests is an encircling suture that is put across the cervix until full term in order to keep it closed.

2. Uterine synechiae – Asherman syndrome

Usually results from destruction of large areas of endometrium during a D&C procedure (Dilatation and curettage). This forms intrauterine adhesions that can cause infertility in women

3. Uterine leiomyomas (Uterine fibroids)

Non-cancerous growths in the woman’s uterus that can cause an extension of the size of the uterus in extreme cases.

4. Abnormal uterus position in body during pregnancy:

This can be of 4 types –

  • Anteflexion: Uterus is normally slightly anteflexed. As a relatively free-floating organ, the uterus tends to normally bend a little forward in the abdomen. There is an increase in the degree of this forward bend to some extent during early weeks of pregnancy. However, some women may have an unnaturally anteflexed uterus, such that it sometimes pushes against the bladder. Such a grossly anteflexed uterus may prevent proper transmission of labor contractions and may cause difficult and/or delayed labour.
  • Retroflexion: In this case, the uterus is bent backwards towards the spine. In this case, it usually pushes against the rectum. Symptoms include abdominal discomfort, pelvic pressure, and painful sex. If sever may cause difficult labour.
  • Sacculation: Persistent entrapment of uterus in the pelvis during pregnancy. It may lead to extensive dilatation in the lower part of the uterus to accommodate the foetus.
  • Uterine torsion: During pregnancy, the uterus rotates on its axis (mostly clockwise, rarely anticlockwise). A small degree of this rotation is normal. But in some rare cases, this rotation maybe more than 45 degrees. This is known as uterine torsion. Cases of upto 720-degree rotations have been recorded. Associated symptoms like obstructed labour, intestinal or urinary complaints, abdominal pain and vaginal bleeding may be observed. Both maternal and foetal complications are reported.

How Does an Abnormal Uterus Affect Your Pregnancy?

The uterine abnormalities, if not mild can have a significant negative impact on pregnancy. Here are some of the common complications that arise with an abnormal uterus:

  • Infertility
  • First- and second-trimester miscarriage
  • Malpresentation (Condition where there is an abnormal position of the foetus)
  • Foetal-growth restriction, foetal demise (leading to still-birth)
  • Preterm delivery because of prematurely ruptured membranes
  • Increased risk of an ectopic pregnancy, a condition where the embryo embeds itself outside the uterus, commonly in the fallopian tubes
  • Uterus pain during early pregnancy
  • Renal problems: 40 percent of women with unicornuate uterus aare likely to have renal (kidney-related) anomalies, and evaluation for these is needed.

Tips to Keep Your Uterus Healthy During Pregnancy

A healthy uterus means a healthy pregnancy. Following are some general guidelines:

1. Work

In the absence of complications, most women can continue to work until the onset of labor (American Academy of Paediatrics and the American College of Obstetricians and Gynaecologists, 2012). Any occupation that subjects the pregnant woman to severe physical strain should be avoided. Ideally, no work or play should be continued to the extent that undue fatigue develops. Adequate amount of rest should be provided especially in a complicated case or if the woman has had complications in a previous pregnancy.

2. Exercise

In general, pregnant women do not need to limit exercise, provided they do not become excessively fatigued or risk injury. Unless specified by the doctor, pregnant women should be encouraged to engage in regular, moderate-intensity physical activity for 30 minutes or more a day.

3. Sea food consumption

Nearly all fish and shellfish contain trace amounts of mercury. Hence pregnant and lactating women are advised to avoid specific types of fish with potentially high methyl mercury levels. These include shark, swordfish, king mackerel, and tile fish. It’s recommended that pregnant women ingest no more than 12 ounces or two servings of canned tuna per week and no more than 6 ounces of albacore or “white” tuna.

4. Travel

The American College of Obstetricians and Gynecologists (2010) has formulated guidelines for automobile passenger restraints use during pregnancy.

  • Women should be encouraged to wear properly positioned three-point restraints (seat-belt) throughout pregnancy while riding in automobiles.
  • The lap portion of the restraining belt should be placed under the abdomen and across the upper thighs.
  • The belt should be comfortably snug.
  • The shoulder belt also should be firmly positioned between the breasts.Available information suggests that airbags should not be disabled when there is a pregnant woman in the car.
  • In the absence of obstetrical or medical complications, pregnant women can safely travel by an airplane up to 36 weeks of gestation. Significant risks with travel, especially international travel, are infectious disease acquisition and development of complications by virtue of being remote from adequate resources. Make sure you are well-informed about these before choosing to travel internationally.

5. Coitus

In healthy pregnant women, sexual intercourse usually is not harmful. But it should be avoided whenever abortion, placenta previa or preterm labour threatens. Intercourse late in pregnancy specifically has not been found to be harmful. However, a case report described a fatal air embolism late in pregnancy as a result of air blown into the vagina during oral-vaginal intercourse. Proceed with caution.

6. Immunization

Most of the vaccines are contraindicated in pregnancy and not a routine unless as pre- or post-exposure prophylaxis in high risk mothers. But two highly recommended vaccines are: Tetanus for every mother (two injections), and Influenza in case of a flu outbreak.

7. Caffeine

It is unclear if caffeine consumption is associated with preterm birth or impaired foetal growth. The American Dietetic Association (2008) recommends that caffeine intake during pregnancy be limited to less than 300 mg daily, or approximately three 150ml cups of percolated coffee.

8. Backache

It can be reduced by squatting rather than bending when reaching down, by using a pillow back support when sitting, and by avoiding high-heeled shoes.

Thus, you can see the role that the uterus plays in pregnancy and how important it is for you to take care of it. Even if you are comfortable with the growing baby inside you, it is still recommended that you do not exert yourself as many factors could lead to complications.

Previous article «
Next article »