Friday June 22 2012
Twins have higher early death risk, statistics suggest
"Twins are five times more likely to die than single babies in their first year of life," The Daily Telegraph has reported. The Daily Mail and The Guardian carry similar headlines stating that twins and triplets are more likely to die in their first year.
The sombre and alarming news headlines are in response to a report from the Office for National Statistics (ONS) that gives data on the number of live births, stillbirths and infant deaths that occurred in England and Wales in 2009. There were 706,248 live births in 2009. Of these, 3,180 infants had died before their first birthday - an infant mortality rate of 4.5 deaths per 1,000 live births.
The report highlights the higher infant mortality rate for multiple births (20.4 deaths per 1,000 live births) compared with single births (4.0 deaths per 1,000 live births). Most of these deaths among multiple births occurred in the first 28 days of life. The report reveals many characteristics of the mothers giving birth in 2009, including their age, ethnicity and socio-economic status. Most of the papers rather surprisingly overlooked these factors, which included greater risks associated with unmarried mothers and immigrant mothers.
The ONS report provides objective data only. The higher mortality rate among multiples compared with single births is likely to be a reflection of risk factors inherent to multiple births. These include the fact that twins and multiples are more likely to be born prematurely and weigh less at birth. The report bears this out. It notes that multiple birth babies were more likely to be of lower birth weight than single births. It also showed that most deaths among multiples occurred in the first 28 days of life – suggesting that the higher infant mortality rate for multiple births may be partly due to pregnancy or birth-related factors. However, the report did not explore the underlying reasons for the higher infant mortality rate among twins, triplets or other multiples.
The media focus upon the higher infant mortality rate among twins and triplets is somewhat alarmist, particularly given the unavoidable risk factors inherent in multiple pregnancies. The media debate about whether IVF is contributing to the mortality rate is a sideline issue. Encouragingly, and in contrast to the stark headlines, the ONS report says that there has been a continual decline in the infant mortality rate over the past 30 years.
What are the main findings of the ONS report?
The report covers all infants who were born in England and Wales during 2009. It provides data on infant mortality (defined as the number of babies who died before their first birthday) and includes infants who died in 2010 but had been born in 2009. It looked at overall mortality, risk factors from being a twin or multiple infant, and risk factors relating to the parents.
Overall infant and perinatal mortality
- There was a total of 3,688 stillbirths in 2009, and 1,694 infants were born live but died before seven days of age. This gives a perinatal mortality rate (deaths around the time of birth) of 7.6 deaths per 1,000 total births (including live births and stillbirths). There were 706,248 live births in 2009, and 3,180 infants died before their first birthday, giving an infant mortality rate of 4.5 deaths per 1,000 live births.
- There was no seasonal variation in the number of live births, stillbirths or infant deaths.
- Conditions related to prematurity or immature development, such as respiratory and cardiovascular disorders, were the most common cause of infant deaths, accounting for 44%. Congenital anomalies were another major cause, accounting for 31% of infant deaths.
Risk for multiple infants
Overall, the infant mortality rate for all multiple births (twins, triplets and higher multiple births) was five times higher than for single births (20.4 deaths per 1,000 live births compared with 4.0 deaths per 1,000 live births). However, most deaths among multiples occurred during the first 28 days of life. This means that the risks at different ages were significantly different.
- In the first 28 days of life, multiples were six times more likely to die than single births (16.1 infant deaths per 1,000 live births among multiples compared with 2.7 per 1,000 among single births). After the first month of life multiple infants were three times as likely to die than single infants (4.3 infant deaths per 1,000 live births compared with 1.3 per 1,000).
- Multiple infants tended to be of lower birth weight than single infants. Just over half of multiple infants with a known birth weight were low birth weight (less than 2,500 grams) and 9.3% of those with a known birth weight were very low birth weight (less than 1,500 grams). By contrast 5.6% of single infants were of low birth weight and only 0.9% were very low birth weight.
- The highest infant mortality rates were for the extremely low birth weight babies (less than 1kg). Among extremely low birth weight single infants, the infant mortality rate was 319.2 deaths per 1,000 live births. Among extremely low birth weight multiples, the infant mortality rate was 391.5 deaths per 1,000 live births.
Parental factors related to infant and perinatal mortality
Interestingly, social and demographic factors relating to the parents were related to infant mortality. These were:
- Among mothers having multiple pregnancies, 63% were aged over 30 years, compared with 47% of mothers of single infants.
- Mothers aged under 20 have the highest infant mortality rate for both single births and multiple births.
- The infant mortality rate for babies born inside marriage was lower than for those born outside marriage.
- The infant mortality rate for single infants was highest for those registered solely by their mother, or registered jointly by parents living at different addresses.
- For multiple births the infant mortality rate was highest for those jointly registered by both parents living at different addresses.
- For married women, the infant mortality rate for single births was higher for women who have previously had three or more children compared with women who have had no previous children.
- For single births, the highest infant mortality rate was found among infants of men who did ‘semi-routine’ jobs.
- The infant mortality rate for mothers born outside the UK was greater than that for mothers born inside the UK.
Why might twins and multiple infants be at higher risk?
The report did not set out to examine all the possible reasons for the higher infant mortality rate among multiples. The higher rate is most likely to reflect the inherent risks associated with multiple pregnancies, including the fact that they are more likely than single births:
- to be born prematurely
- to have intrauterine growth restriction
- to be of lower birth weight
- to have congenital abnormalities
- to have complications around the time of labour and delivery, such as umbilical cord prolapse or premature separation of the placenta
The risks of each of these complications increases with the number of infants in the pregnancy.
These possible explanations are in part borne out in the report, which notes that:
- multiples were more likely to be of lower birth weight than single infants
- most deaths among multiples occurred in the first 28 days of life
- most cases of infant mortality were related to immaturity
This suggests that the higher infant mortality rate among multiples may have been, at least in part, due to pregnancy and birth-related factors. How much each of these factors played a part cannot be judged from these statistics.
Though the researchers explored possible associations with maternal and socio-economic factors they note that, compared with single infants, the absolute number of infant deaths occurring among multiples is small. This means that analysis of the causes of early mortality of twins and multiples and other factors will be less robust.
Is IVF contributing to the higher rate of twins and multiple births?
Much of the media has connected the problems with multiple births with IVF. The only reference to IVF made in the ONS report is that it says that multiple births are more common among women aged over 30, and this may be due to the rise in the use of fertility treatments among women who have delayed childbearing for personal or social reasons. It quotes a 2007 Human Fertilisation and Embryology Authority (HFEA) report on multiple births after IVF. It says that around one in four IVF pregnancies currently result in the birth of twins – more than 10 times more than the naturally occurring twin birth rate. This twin birth rate after IVF needs to be brought down to a rate of 10% of all births resulting from IVF for health reasons, the HFEA says.
It’s important to bear in mind that the ONS report focuses on the rate of infant deaths, not the reasons for the increase in multiple births.
Analysis by Bazian. Edited by NHS Choices.