The extraordinary pregnant body (part 2)

The Uterus

While the outer appearance of the ‘baby bump’ is one of the most recognised signs of pregnancy, it’s vital to understand what is happening on the inside.

The uterus expands with each week of gestation to accommodate the growth of the baby. It’s primary function is to nurture and keep your little one safe, so it is important to understand its structure and function, and what changes you can expect the uterus to undergo during pregnancy.

What does the uterus look like?

A uterus is the medical term used for the female womb which is a pear-shaped muscular organ measuring approximately 7.5cm long by 3.5cm wide.

During the female menstrual cycle, the body secretes hormones that cause ovulation (when the egg is released from the ovary) and the monthly period (menstruation). The lining of the uterus is called the endometrium (made of several layers that include surface epithelium, blood vessels, glands and other tissues). Each month the endometrium grows thicker to prepare for pregnancy.

If a woman does not become pregnant, the top layers of the endometrium are shed causing menstrual bleeding. If conception occurred, the uterus will then provide structural integrity for the fertilised ovum as it becomes an embryo and develops into a foetus.

The uterus is where the foetus (the baby) will remain for the duration of the gestation period.

What functions does the uterus have during pregnancy?

The uterus plays an incredibly important role during pregnancy, which includes housing and nourishing your unborn baby.

The most important of these are:

• Providing a suitable environment for your developing baby
• Raising progesterone levels which promote pregnancy success by preventing miscarriage
• Producing the right levels of oxytocin, required for labour and breastfeeding
• Contraction of uterine muscle fibres which provide a rhythmic ‘milking’ action that helps move the baby through the birth canal during labour

What changes does your uterus undergo during pregnancy?

There are several anatomical and physiological changes that take place in the uterus as pregnancy progresses; these can be divided into those related to size, those related to shape and those related to the muscular activity of the uterus.

Size

The uterus increases dramatically in size during the nine months of pregnancy (it grows from approximately 7.5cm long before pregnancy to around 20-30cm long at term), with the greatest rate of growth in the first trimester.

The growth in your uterus is due to hypertrophy (increased muscle mass) and hyperplasia (increased numbers of cells).

Shape

The uterus becomes more spherical in shape, with an increase in myometrial thickness. This helps to ensure that labour can be carried out effectively. The cervix becomes soft and more ‘open’, which helps the baby pass into and through the birth canal.

Even though your uterus grows dramatically during pregnancy, it should return almost completely back to its pre-pregnant state, approximately 4 to 6 weeks after delivery.

How does the uterus prepare for labour and birth?

Some women may experience contractions as early as mid-way through their pregnancy, continuing all the way to the birth. The ‘practice contractions’ are widely known as Braxton Hicks contractions or 'false labour'. These contractionsis your uterus preparing for the birth of your baby. Braxton Hicks contractions tend to be fairly irregular and not generally painful. If you experience these, it’s always recommended to speak to your healthcare team so that they can ensure everything is going well with the baby.

At the onset of true labour, the muscles of the uterus contract which helps your baby move down into the birth canal. Unlike Braxton Hicks contractions, labour contractions become progressively stronger and more frequent until the birth. Some women have described these as a wave, moving from the top of the uterus down to the cervix.

How does the uterus change postpartum?

After the birth of your beautiful baby, your uterus will contract again in order for the placenta to leave the body (this is called the afterbirth). The uterus will stay contracted after this to assist in the prevention of postpartum haemorrhage (heavy bleeding).

During breastfeeding the uterus will continue to have contractions which may cause mild cramps for some. This contracting and tightening of the uterus will feel a little like period cramps and is also known as 'afterbirth pains'.