Written by : Dr.M.D.Mazumdar, MD
The process by which the uterus and the other genital organs return to their normal pre-pregnant state in the postpartum period after the delivery of the fetus is called 'involution'.
The uterus not only returns to it's pre-pregnant size, but it also returns to its normal position in the pelvic cavity and regains its muscular tone. The vagina, uterine ligaments and muscles of the pelvic floor all revert back to the pre-pregnant state.
Involution of the Uterus
During the pregnancy, the uterus enlarges due to growth and stretching of the uterine muscles. The muscles not only increase in size but also in number.
Immediately after the delivery of the baby, the uterus has two main aims - the first is to control bleeding and the next is to revert back to its former state.
Bleeding is controlled by contraction of the muscles. The uterine muscles are arranged in criss-cross layers - so that contraction of the muscles constricts the blood vessels between them, causing less blood flow. This is the early stage of involution.
At this time, it forms a firm, ball-like structure just below the bellybutton. It is tender and pressing on it can cause pain.
Involution starts by autolysis and phagocytosis. Autolysis, more commonly known as self-digestion, refers to the destruction of a cell through the action of its own enzymes. During involution, the muscles undergo some amount of autolysis to reduce in size.
Autolysis occurs in response to the withdrawal of placental hormones after the childbirth. During pregnancy, hormones like estrogen and progesterone help the uterine muscles to enlarge and grow rapidly. Blood vessels too increase in number and size.
The gradual decrease in size of the uterus during involution occurs due to a decrease in size but not a decrease in the number of uterine muscles. This causes the uterus to remain somewhat larger in size than the pre-pregnant state. The size of a uterus which has once undergone pregnancy is larger than a uterus which has never gone through pregnancy.
After involution is complete at around 6 weeks postpartum, the uterus is more or less its normal size but the endometrial cavity remains larger than in the virgin state. The blood vessels are also more tortuous, and its muscular layers are more defined.
The uterus, which weighs about 900 grams at the end of labour, weighs only about 40-60 grams at the end of the postpartum period, 6 weeks after childbirth.
Rate of Involution of the Uterus
Immediately after the delivery, the upper margin of the uterus lies about 5 inches above the pubic symphysis (upper margin of the pubic bones). In an average sized woman, this will be at the level of the umbilicus.
The rate of involution is maximum in the first five days (about 1 cm per day) of the postpartum period and then gradually slows down. By the 7th day, the uterus becomes much smaller and only its upper border can just be felt at the level of the symphysis. It regains its pre-pregnancy size at the end of 6 weeks.
Involution is accompanied by the flow of Lochia - a thick vaginal discharge consisting mainly of the thick endometrium formed to help support the pregnancy.
Rate of Involution of the Uterus
Involution of the Cervix
The cervix involutes more slowly than the uterus. Immediately after the delivery of the baby, it is a loose opening with irregular edges. It is soft and toneless with deep fissures along its edges.
But by the end of the first week, it becomes more clearly defined, regaining its canal like structure. Its opening in the vagina now is much smaller and the internal os can admit only the tip of the finger.
While the internal os, which is the opening of the cervix near the uterus involutes completely, the external os, which is the outer opening of the cervix in the vagina, never regains its pre-pregnant state. It remains slightly open and its edge always presents one or more fissure.
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Involution of the Vagina
The vagina involutes more slowly than the uterus. Immediately after the delivery, it is swollen, toneless and appears bruised and red. The normal rugosity (wrinkles) of the vaginal walls reappear at about the 3rd week of the postpartum period. But the size and elasticity of the tissues never regain the pre-pregnancy state.
Taking Sitz baths and regular Kegel's exercises help heal the vagina and tone the vaginal muscles faster.
Other Pelvic Organs
Other pelvic organs like the ligaments of the uterus, muscles of the perineum, ovaries and fallopian tubes also involute and completely regain their normal appearance by the end of 6 weeks after delivery.
Measurement of the Rate of Involution
The height of the uterus is measured from the upper edge of the pelvic brim to the upper level of the uterus with a tape and a record should be kept if there are any problems with normal involution of the uterus.
Subinvolution is an abnormality in the involution of the uterus in which the rate of involution is lesser than normal. Causes of Subinvolution are:
- Cesarian Section - this causes delayed healing of the incised uterine muscles.
- If the uterus is not completely empty after the childbirth and there is retained products of conception like bits of the placenta or membranes, involution may be hampered. These can also form the focus of infection and cause postpartum infection.
- Prolonged labour exhausts the uterine muscles and delays healing, leading to subinvolution.
- Multiple pregnancies like twin or triplet pregnancies cause excessive stretching of the muscles fibres and these may take longer than normal to come back to the normal state.
- Secondary infection - Postpertum Infection can delay healing and involution.
- If the bladder is not completely emptied, the uterus may appear to be at a higher level than normal and a wrong diagnosis of subinvolution made. The uterus should always be measured after the bladder has been emptied.
Signs and Symptoms of Subinvolution
The main sign of subinvolution is delayed decrease in the size of the uterine height. Lochia Rubra is present for a longer time than normal. Infection is indicated by a foul smell of the lochia.
Treatment of Subinvolution
Treatment is according to the cause.
- Chronic diseases : Chronic diseases like tuberculosis can cause subinvolution. The tuberculous toxins appear to have a special affinity for the ovary and the uterine endometrium.
- Nutritional Causes : Severe starvation as seen in malabsorption syndromes and during war and famine can cause subinvolution after childbirth.
- Infection : Postprtum infeciton is a very common cause of subinvolution.
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