Caesarean risk to womb

Having a caesarean section for the birth of your first baby increases the risk of a ruptured womb when you try to have a second baby, reported The Times and other newspapers. They say that “mothers who try to have their second child naturally are 50 times more likely to suffer a ruptured womb during childbirth if they’ve previously had a Caesarean”, the newspaper said. A ruptured womb (uterine rupture) can be life threatening, and, according to the newspaper stories, result in the death of one in 20 babies.

The stories are based on a study that looked at the outcomes of second pregnancies and found that the risk of rupture to the uterus is increased by previous caesarean section, although the actual number experiencing ruptures in both groups was small. The fact that a previous caesarean increases the risk of uterine rupture in a natural birth is well recognised by the medical profession; many women deliver their subsequent children by caesarean if they delivered this way previously.

Where did the story come from?

This study was conducted by Melissa Kaczmarczyk and colleagues from the Department of Epidemiology at Emory University, Atlanta, Georgia, USA and the Department of Medical Epidemiology and Biostatistics at the Karolinska Institute, Stockholm, Sweden. Funding was provided by grants from the Karolinska Institute and it was published in the peer-reviewed medical journal BJOG: An International Journal of Obstetrics and Gynaecology .

What kind of scientific study was this?

This was a cohort study in women attempting vaginal birth in their second delivery. It was designed to look at the risk factors for uterine rupture (torn uterus) and problems to the newborn when a normal birth was attempted after the delivery of a previous baby by caesarean section.

The researchers used the Swedish Birth Register to identify 300,200 women who had a first baby born after 1983 and a second live single birth born between 1992 and 2001. For each woman, they looked at whether the first delivery was normal or by caesarean, the time interval between first and second pregnancies, the onset of second delivery (whether labour was spontaneous or induced), whether the second delivery was normal or by caesarean, and information about the second baby’s birth weight. Other information about the mother was also considered. This included age, weight, and whether she was a smoker.

Statistical methods were used to examine the relationship between uterine rupture and several possible factors, such as method of delivery, whether birth was induced, interval between pregnancies. The researchers also looked at how the risk of infant death (within 27 days of live birth) was related to uterine rupture and to other factors.

What were the results of the study?

All women in the sample attempted a normal birth, however, 4.1% of women had their second baby by caesarean. 24.7% of women who had their first baby by caesarean required an emergency caesarean for their second baby compared with 2.2% of women who had given birth normally the first time.

Overall, there were 274 cases of uterine rupture at second birth (an overall rate of 0.91/1,000 women). In women who had had their first baby normally, uterine rupture was seen in 0.18 in every 1,000 women. In women whose first delivery was by caesarean, nine in every 1,000 women experienced rupture. This means that women who had a previous caesarean delivery were 42 times more likely to have a uterine rupture if they attempted a normal birth with their second baby.

The researchers also found that women who needed to have their labour induced had double the risk of uterine rupture compared with women who went into labour naturally, however this risk increase was the same regardless of how the first baby was born.

Other factors that gave small increases in the risk of uterine rupture, considered separately from the previous method of birth, were maternal age over 35, short maternal height (less than 5ft 2in/159 cm), higher birth weight of the baby (more than 4kg/8.8lbs), and gestational duration of more than 42 weeks.

Death of the infant was more likely if women had a uterine rupture. The infant death rate was 51.09 for every 1,000 cases of delivery with uterine rupture, compared with 1.4 deaths for every 1,000 deliveries without rupture.

What interpretations did the researchers draw from these results?

The researchers concluded that having a “caesarean section at first delivery was the strongest predictor of uterine rupture” during attempted vaginal delivery at second pregnancy. They also say that other factors have an influence, notably if labour is induced, as increased contractions can put additional strain on a previously scarred uterus. They say that “as the rates of caesarean section and induction of labour continue to rise in developed countries, the number of women at risk for uterine rupture is also increasing.”

What does the NHS Knowledge Service make of this study?

This is a study of a very large number of women that provides reliable data on the rates of uterine rupture. However, this is not a new medical finding; previous caesarean section has always been recognised as one of the potential risk factors for uterine rupture. However, this study does provide a better understanding of the size of the risks associated with previous caesarean section. There are several important points to consider:

• Although the risk of uterine rupture when attempting vaginal delivery may be increased by having had a previous caesarean section, uterine rupture is still a rare occurrence and the actual risk to the individual remains very small.
• This study has not examined many other medical or maternal complications that may be involved in these pregnancies, it only looked at a limited number of factors for which information was available. 
• This study used a registry to identify cases of uterine rupture. The authors acknowledge that this would not have been able to differentiate cases of complete rupture of the uterus from those of partial rupture or uterine scar dehiscence (a break in only certain layers of the uterus without a full tear, and therefore lower risk to the mother and infant). The inclusion of these cases may have increased the rates of ‘uterine rupture’ that the researchers counted in this study.
• It is important to realise that doctors are fully aware of the risks associated with attempted vaginal delivery following previous caesarean and will discuss the most appropriate delivery plan for each individual pregnant woman. Many women who have had a previous caesarean delivery will plan another caesarean delivery, thereby minimising any risk.

Sir Muir Gray adds...

This information is scarcely news; I learned this 40 years ago. What is news is that women who have had a previous caesarean section do not necessarily have to have a caesarean for their second child. An excellent study published in the British Medical Journal this year showed that if women were given all the facts clearly, some chose to try for a vaginal delivery and were pleased to have been given full information, and treated as adults.