SIDS in Twins

As a parent who lost one of a set of twins to SIDS in 1987, I had many questions about the relationship of multiple births to SIDS. Much of the literature I acquired from SIDS organizations and other sources had little to say about SIDS in twins although some of it reported that twins are at an increased risk for SIDS. Immediately after the death, my biggest concern was whether my surviving twin would also die. The statistics that I read for both twins dying of SIDS varied widely. I wondered whether the increased risk for twins could be explained by factors known to be associated with SIDS such as low birth weight and prematurity, or whether there are factors exclusive to multiple births that increase the risk. In 1991, I decided to research the topic of SIDS in twins and prepare an article for the Center for Loss in Multiple Birth, Inc., both for my personal interest and to provide thorough, up-to-date information for others affected by the loss of a twin to SIDS.

For the original 1991 version of this article, I reviewed all the literature on SIDS in medical journals between 1970-1991, and found several articles that dealt with SIDS in twins, along with two studies that used twin infants as subjects. In 2003, I decided to update this article by including information from any additional articles on SIDS in twins written after 1991. I found two articles: one (Malloy et al., 1999) performed an extensive statistical analysis of the US birth and infant death records from 1987-1991, and another (Koehler et al., 2001) reviewed cases of simultaneous SIDS in twins. I also am including a summary of important advances in SIDS research that have resulted in a reduction in the overall rate of SIDS, both for singleton and multiple births.

Advances in SIDS Research

In the late 1980s, studies from New Zealand and other countries showed that even though the underlying causes of SIDS were not understood, the rate of SIDS could be significantly reduced by changes in selected child-care practices which included:

• placing infants on their backs to sleep.
• providing a safe sleeping environment. Infants should be placed on their backs in cribs with firm mattresses, and should not sleep on sofas, pillows, soft mattresses, waterbeds, fluffy quilts, and other soft, hazardous surfaces.
• avoid overheating by dressing the infant lightly and keeping the room temperature comfortable.
• avoid exposure to cigarette smoke.
• use breastfeeding.

In 1992, the American Academy of Pediatrics (AAP) issued recommendations that healthy infants be placed on their backs to sleep and in 1994, initiated a public health measure known as the “Back to Sleep” campaign, also supported by the National Institutes Child Health and Human Development (NICHD). Other studies by the U.S. Consumer Products Safety Commission (CPSC) linked soft bedding products to infant deaths, and in 1995, led the CPSC to become active in public education efforts to promote “crib safety.”⁵ By 2000, these public health efforts resulted in a 60% drop in the rate of SIDS, from about 1.5 SIDS deaths per 1000 live births in the 1980s to a rate of .62/1000.²

Comparison of Rate of SIDS in Twins to Singletons

In this section, I will summarize the figures for the overall rate of SIDS in twins and singletons that have been reported in some studies. It should be noted that since singletons are born much more often than twins, the data for SIDS deaths associated with singleton births is based on a much larger sample than that for twin births. The most current study (Malloy, 1996) obtained data from the US-linked birth and infant death records of 1987-1991 collected by the National Center for Health Statistics. It should be noted that this data provided a much larger sample than the earlier studies. I have included the sample
sizes in Table 1.

Table 1.

	Singletons			Twins			Relative Risk
Study	Number SIDS	Number Births	Rate per 1000	Number SIDS	Number Births	Rate per 1000	Twin to Singleton
1968 CaliforniaLive Birth Cohort¹⁰	490	334,326	1.5	25	6,921	3.6	2.4
1974 upstateNew York ¹²	171	129,097*	1.32	13	2,554*	5.09	3.8
1976-1978Wisconsin ¹¹	365	202,163	1.81	14	4,000	3.56	2.0
US Birth & DeathRecords 1987–1991⁴	23,464	19,993,791	1.2	1,056	425,941	2.5	2.1
US Birth & Death Records 1997-2000¹			.62**²	535	461,465	1.16	1.9

*
These numbers are the total number of infants alive at 7 days. The rate of SIDS reported in the upstate New York study would tend to be higher
compared to the other studies because Standfast divides by the number of infants alive at 7 days rather than the total number of live births as in the other studies.

** This value is for the year 2000 only.

The data in this table shows that overall “relative risk” that a twin infant will die of SIDS is about twice that of a singleton.

Comparison of Population Characteristics

I. Comparing Twins to Singletons

Malloy (1999), using data from the US birth and death records of the year 1991 only, examined the distribution of population characteristics between twin births and singleton births, and computed the relative risk of SIDS for a twin infant compared to a singleton for each characteristic. The characteristics he analyzed were race, maternal age, maternal education level, birth weight, and gestational age as shown in Table 2.
The relative risks between twin and singleton births calculated for each strata of population characteristics showed twins to be at greater risk for SIDS for all characteristics except for birth weights between 500 and 2500 grams and gestational age < 32 weeks.⁴ It would be interesting to see how the population characteristics and rates of SIDS shown in Table 2 have changed since the 1992 AAP recommendations of placing infants on their backs to sleep and providing a safe sleep environment.

Table 2.

US Infant Birth & Death Records of 1991

	Singletons		Twins		Relative Risk
Characteristic	% of Singleton Births (n = 3,963,939)	SIDS deaths (n = 4,670) Rate per 1000	% of Twin Births (n = 91,957)	SIDS deaths (n = 211) Rate per 1000	Twin to Singleton
Race
White	79.0 %	1.0	77.6 %	2.0	2.00
Black	16.4 %	2.1	19.1 %	3.4	1.64
Other	4.6 %	1.0	3.3 %	1.6	1.56
Maternal age, y
< 19	8.4 %	1.3	4.7 %	3.6	2.64
19 – 35	84.8 %	1.2	86.1 %	2.3	1.86
> 35	6.7 %	0.6	9.2 %	1.7	2.57
Maternal education level, y
<12	26.2 %	2.0	21.81 %	4.5	2.24
12	36.4 %	1.2	35.6 %	2.4	1.97
>12	37.4 %	0.6	42.6 %	1.1	1.76
Birth weight, g
500 -1499 g (1.1 -3.3 lb)	0.9 %	4.7	8.4 %	4.5	0.97
1500 -2499 g (3.3 -5.1 lb)	4.9 %	3.4	42.5 %	3.1	0.93
>2500 g (5.1 lb)	94.2 %	1.0	49.1 %	1.3	1.23
Gestational age, wk
<32	1.5 %	5.4	9.8 %	4.3	0.81
32 -37	14.8 %	1.9	54.3 %	2.4	1.21
>37	83.7 %	1.0	35.9 %	1.7	1.74

II. Computing the “Relative Risk” of SIDS

The relative risk that a twin will die of SIDS compared to a singleton is defined as the ratio of the death rate of a twin to the death rate of a singleton. In Table 2, the relative risk of a twin to a singleton ranges from 0.81 for the characteristic of gestational age < 32 weeks, to 2.64 for maternal education < 19 years. The values in Table 2 are the “crude” relative risks which reflect the overall risk that a twin will die compared to a singleton in each category. Since the distribution of population characteristics is different for twins and singletons (e.g. 94.2 % of singletons have birth weights greater than 5 lbs. compared to 49.1% of twins), Malloy carried out a statistical analysis (using multivariate logistic regression) and computed the “adjusted” relative risk of a twin dying from SIDS compared to a singleton to be 1.13. The factors adjusted for in the analysis were birth weight, race, maternal age, and maternal education level. This result can be interpreted to mean that the actual risk to twins and singletons is about equal, and the reason that the overall rate of SIDS for twins is higher is because a greater proportion of twins are born in high-risk categories.⁴

III. Comparing Pairs of Twins in which Both Survived to Twins with a SIDS Death

Malloy (1999) computed statistics for the racial distribution, level of maternal education, and sex concordancy for a sample of 767 twins in which at least one infant died of SIDS and compared it with a set of 171,262 twin infants where both of the pair survived. He also computed the mean and standard deviation of the birth weight, birth weight discordancy, and gestational age. The birthweight discordancy is defined as the percentage difference in weight between a twin and its co-twin. The data sample is a subset of the 1987-1991 US infant birth and death records in which twin pairs were included only in cases where a twin infant could be linked with its co-twin. Among twin pairs with a SIDS death, a higher proportion were Black, had mothers with less than 12 years of education, and were part of a female-female pair than twin pairs in which both infants survived. Also, the pairs of twins with a SIDS death had a lower mean birth weight, a larger percentage difference in their birthweights, and younger gestational age. This data is summarized in Table 3.⁴

Table 3.

US Infant Birth & Death Records 1987 – 1991 (only linked co-twins included)

Characteristic

Pairs of twins in which both twins survived

(n = 171,262)

Pairs of twins with at least one SIDS death

(n = 767)

Race White

Black

Hispanic

78.4 %18.2 %

3.⁴ 2 %

67.9 %28.5 %

2.67 %

Maternal education level, yr < 12

> 12

40.3 %27.8 %

31.9 %

50.1 %32.3 %

17.6 %

Sex concordancy Male – male

Female – female

Male – female

34.9 %34.6 %

30.5 %

27.6 %41.3 %

31.0 %

Birth weight of pair gr, (mean +/ – sd*)

2491 +/ – 547 gr

2225 +/ – 549 gr

Birth weight discordancy percentage, (mean +/ – sd*)

11.2 +/ – 8.9 %

12.5 % +/ – 10.7 %

Gestational age wk, (mean +/ – sd*)

36.4 +/ – 3.2 wk

35.7 +/ – 3.6 wk

*mean – the average of a sample of data

standard deviation (sd) – a statistical measure of the spread of a sample of data

Discussion of Risk Factors

I. Prematurity and Birth Weight

Low birthweight and prematurity are frequently mentioned in the literature as increasing the risk for SIDS, and infants resulting from multiple births are often subject to these problems.

One study, the 1968 California live birth cohort (Kraus, 1983), shows that low birthweight twins have about twice the risk for SIDS than low birthweight singletons¹⁰, but this result is based on a sample size of only 25 SIDS deaths and is not borne out in the larger 1991 study (Malloy, 1999) shown in Table 2. The 1991 study shows that while low birthweight infants and premature infants have a higher rate of SIDS than infants weighing over 2500 gr. (5.1 lb.) at birth, the risk is similar for both twins and singletons⁴.

II. Birth Order

Some studies have shown that the second-born twin is more likely to die of SIDS. The study (Standfast, 1980) using data from 1974 in upstate New York analyzed the birth and death certificates of babies born during that time. In 2,725 twin births, 4 first-born and 9 second-born twins died. The risk to first-born twins was 3.09 /1000 and to second-born twins was 7.15 /1000. Standfast points out that the risk of death to the second-born twin is higher from other causes during delivery and the perinatal period. She suggests that second-born twins are more likely to be subject to perinatal hypoxia which may be a risk factor for SIDS.¹²

Similar findings were reported by Getts (1981) in a study on SIDS deaths in Wisconsin from 1976 to 1978. In nearly 4,000 twin births, 3 first-born and 11 second-born twins died of SIDS for respective rates of 1.53 /1000 and 5.6 /1000.¹⁰

In a study (Beal, 1989) of a sample of 37 twin infants who died of SIDS in Adelaide, Australia, from 1973 to 1988, 15 were first-born and 22 were second-born.⁶

Two studies do not support the finding that second-born twins are more likely to die of SIDS. Kahn (1986) studied a sample of 42 twin infants who died of SIDS of which 25 were first-born and 17 were second-born.⁸
Also, a study in England called the Oxford Record Linkage Study reported 14 twin deaths in which 9 were first-born and 5 were second-born.¹⁰ The study by Malloy (1999) shown in Table 1 did not report whether the twins were first- or second-born.⁴

III. Difference in Birth Weight Between the Infants

One of the factors that Beal (1989) examined was if one twin was significantly smaller than its sibling at birth. Beal considers one twin to be significantly “growth retarded” compared to the co-twin if the difference in birth weights is more than 15 %. In Beal’s study of 37 twin infants who died of SIDS in Adelaide, Australia from 1973 to 1988, she found that in 14 of the pairs of twins in which one died, one sibling was more than 15 % heavier at birth than the other. In 12 of these pairs, it was the smaller twin who died.⁶

Malloy (1999) computed the mean and standard deviation of the birthweight, birthweight discordancy, and gestational age (see Table 3) for a sample of 767 twins in which one infant died of SIDS and compared it with a set of 171,262 twin infants where the both of the pair survived. The data shows that the sample with a SIDS death had a lower mean birth weight, a larger percentage difference in their birth weights, and younger gestational age. Malloy (1999) does not report whether the larger or smaller twin was more likely to die.⁴

IV. Other Risk Factors for SIDS

Kahn (1986) studied 42 twin pairs in which one twin died of SIDS and compared the data with a control group of 42 age- and sex-matched pairs of twins, both of whom survived the first year. He hoped that comparing the histories of the co-twins would shed some light on risk factors for SIDS, since the co-twins share similar socioeconomic and parental backgrounds. Information was collected on all of the twin pairs from a questionnaire, parental interviews, and checking medical records. Of 114 items studied, only 11 variables were found to be statistically significant between the SIDS victim and surviving co-twin. These 11 items were then compared between the SIDS victim and the 42 pairs of control infants in which both twins survived the first year. Only 4 of the variables were found to be statistically significant between the SIDS victim and the control infants, which were weight at birth, length
at birth, cyanosis or pallor during sleep, and profuse sweating during sleep. These results are summarized below in Table 4.⁸

Table 4.

Comparison of Characteristics of Twin Pairs in which One Twin Died of SIDS

to Control Twin Pairs

Characteristic	SIDS Infant	Surviving Twin	Control Infants
Wt at birth (g)	2,300	2,450	2500
Length at birth (cm)	44	45	45.9
Stay in nursery (d)	23	19
Age when follows moving object (wk)	6.5	5.8
Age when begins to smile (wk)	7.5	6.7
Age when controls head (wk)	9	8
Fatigue during feeding (No.)	11	2
Reduced arm and neck tonus (No.)	9	1
Cyanosis or pallor during sleep (No.)	4	0	0
Longer sleepers (No.)	8	1
Profuse sweating during sleep (No.)	9	0	0

The Risk of Losing Both Twins to SIDS

An immediate concern of a family that has lost one twin to SIDS is that the other baby may also die. It is of interest for the family to know the probability that the co-twin will die of SIDS given that one member of a twin pair has succumbed to SIDS. This statistic is sometimes called the “rate of concordancy” in the literature.

Malloy (1999) analyzed a sample of 767 twins in which one (or both) infants died of SIDS and compared it with a set of 171,262 twin pairs where both of the pair survived, giving an overall SIDS rate of 2.2 per 1000 births. The data sample is a subset of the 1987-1991 US infant birth and death records in which twin pairs were included only in cases where a twin infant could be linked with its co-twin (the same data sample used in Table 3). Of the 767 twin pairs with SIDS deaths, there were 7 in which both twins died of SIDS, giving a “rate of concordancy” of 0.9. Of the 7 pairs, there was only one case where both twins died on the same day (simultaneous). From this data, summarized in Table 5, the rate of both twins dying of SIDS can be computed as 4 per 100,000 twin pairs (or .004 per 1,000 twin pairs), and the rate of both twins dying on the same day is .58 per 100,000 twin pairs (or .00058 per 1,000 twin pairs). Another statistic, the “relative risk” of a second twin dying given that one twin had already died of SIDS can be computed as 8.17, meaning that the second twin is 8.17 times more likely to die than a twin whose co-twin did not die of SIDS.⁴

Table 5.

US Infant Birth & Death Records 1987 -1991 (only linked co-twins included)

Twin Pairs Where One of Both Infants Died of SIDS by Year of Birth
Twin Pairs	1987	1988	1989	1990	1991	Total
One twin died of SIDS	117	110	172	189	165	753
Both twins died of SIDS (simultaneous deaths*)	3 (0)	1 (0)	0 (0)	0 (0)	3 (1)	7 (1)
Both twins died but SIDS caused only 1 death	0	0	1	3	3	7
Total twin pairs with at least one SIDS death	120	111	173	192	171	767

* number of simultaneous deaths is in parentheses

Malloy (1999) then examined several characteristics summarized in Table 6 of the 7 twin pairs in which both twins died of SIDS. He reports that all but one of the pairs were concordant for sex, the mean weights of the twin pairs were less than the mean weight of twin pairs in which only one twin died of SIDS, and the mean gestational age was higher than that of twin pairs in which only one twin died of SIDS. Based on a standard reference for fetal growth, all 7 of these twin pairs were small for gestational age.⁴

Table 6.

Characteristics of Twin Pairs in Which Both Twins Died of SIDS

Pair	Race	Sex	Gestational age, wk	Birth Weight, gr	Age at Death, wk	Difference in Age at Death, wk
1	White	MaleMale	3939	11551240	1613	3
2	White	FemaleFemale	3939	17861899	1816	2
3	White	FemaleFemale	3535	14402160	126	6
4	Black	MaleFemale	3737	20412296	821	13
5	White	MaleMale	3636	19652205	2020	0 (simultaneous)
6	Black	FemaleFemale	3838	16161928	517	12
7	Black	MaleMale	3939	22962325	114	7

In an earlier study, Beal (1989) reviewed a series of 23 studies dating from 1956 through 1988. In order to obtain a large sample of twins
affected by SIDS, she simply combined the data from these 23 smaller studies. Together, these studies recorded a total of 637 twin births in
which at least one twin died of SIDS, and the death or survival of the co-twin was also reported. Of these 637 twin pairs, there were 619 pairs
in which one twin died of SIDS and 18 in which both twins died of SIDS giving the rate of concordancy of 2.8 %. Of the pairs in which both
twins died, 12 died simultaneously (2) and 6 of the surviving co-twins died later (1 ). Thus, in Beal’s study, it was more common for pairs of
twins in which both died of SIDS to die simultaneously (within 24 hours) than separately.⁶

The probability that a co-twin will die of SIDS given that its twin has already died of SIDS (the “rate of concordancy”) in these studies is 0.9 % in Malloy’s study, and 2.8 % in Beal’s study. In my opinion, Malloy’s estimate of 0.9 % is more accurate since it was based on a large sample from a single source
rather than by combining many small samples which individually may not have been statistically representative of the populations.

Simultaneous SIDS in Twins

Koehler (2001) searched the literature from all over the world between 1900 and 1998 in an attempt to locate all recorded cases of simultaneous SIDS. He found 41 cases cited in the world literature and evaluated them to see if they met the definition of SIDS, analyzing information such as the location and circumstances of the deaths. The official definition of SIDS set forth by the National Institutes of Child Health and Human Development (NICHD) is: the infant must be between 7 and 365 days old,
and no cause of death is identified by autopsy, toxicology screen, medical history of the infant and mother, or investigation of the death scene. Koehler proposed 3 criteria for twins’ deaths to be considered simultaneous SIDS:

1. both infants must independently meet the definition of SIDS set forth by NICHD.
2. the infants must be members of either a monozygotic or dizygotic pair.
3. simultaneous implies that the deaths must occur within 24 hours of each other.

Of the 41 cases, Koehler found only 12 pairs that met all 3 criteria (29.2 %), nine pairs met 2 criteria (21.9), an alternative cause of death was proposed in 5 pairs (12.1 %), and due to limited information on the remaining 15 cases (36.6 %), no conclusion could be reached. In one of the cases, one of the authors actually conducted the autopsy and reviewed the medical history. Koehler concludes that a meaningful estimate of the incidence of simultaneous SIDS cannot be determined unless all the relevant information is recorded for the cases.³

Studies Using Twin Infants as Subjects

A researcher, Jeffrey Gould, conducted two studies where twins were used as subjects. He hoped that studying twins would reveal some clues into the causes of SIDS because twins are a “high-risk” group for SIDS. His studies are summarized below:

I. The Relationship between Sleep and Sudden Infant Death

Gould (1988) conducted a study to examine “sleep-state maturation” in infants. As subjects, he used a set of 29 twin infants and 24 singleton infant controls. The reason that he selected twins as subjects is that several studies have reported twins to be at high risk for SIDS. He also considered risk factors such as race, sex, and gestational age in the analysis of his data. All of the babies studied were healthy, and none subsequently died of SIDS.

Gould is interested in the relationship between sleep and SIDS because of “evidence to suggest that in infants at risk for SIDS, mechanisms essential for the maintenance of homeostasis during sleep may be compromised”. The article explains that “sleep is a complex neurophysiologic process consisting of the
alteration of two behavioral states, quiet sleep, and active (REM) sleep”. The newborn has “a preponderance of REM sleep”, but over the first three months, the percentage of quiet sleep increases and the infant’s sleep pattern changes from a series of multiple brief naps into the more prolonged adult pattern. This change in sleep pattern is accompanied by the development of interconnections in the brain between higher centers and the brain stem. This integration is believed to be essential “for arousal and cardio-respiratory homeostasis”, and Gould theorizes that “it is the interplay between the homeostatic demands of prolonged sleep and the ability of the maturing infant to meet these demands” that may be a factor in SIDS. This theory could explain the age distribution of SIDS deaths, which reach a peak at 2-3 months.

“Polygraphic” sleep studies measuring several physiological quantities were conducted every few weeks when the infants were between 0-3 months old. These measurements allowed 3 categories of sleep state to be determined, REM, quiet, and “intermediate”.

Comparisons were made between the “sleep-state organization” of the infants at high risk (twins, Black
twins, male twins, premature twins) and those at lower risk (singletons, White twins, term twins). A major finding of the study was that the groups at high risk tended to have a decreased percentage of quiet sleep which reflects a decreased level of maturation in these groups. Gould (1988) theorizes that chronic intrauterine and postnatal hypoxia could be responsible for the decreased maturation of quiet sleep, which could in turn affect cardiorespiratory control and arousal mechanisms in the infant. More sleep research is needed to understand the relationship between homeostasis, arousal, and the development of the infant.⁷

II. Apnea and Sleep State in Infants with Nasopharyngitis

Gould (1980) studied the effect of mild upper respiratory infections (colds) on sleep by analyzing 182 sleep polygraphs performed on newborn twins, a “high-risk” group for SIDS. The twins were between 40 and 52 weeks “post-conception” (i.e. about 0-3 months old). In 30 % of the studies, the infants had symptoms of a mild upper respiratory infection. Gould was interested in both alterations in the sleep state cycle (REM, quiet) and in apnea episodes recorded during sleep. Apnea during sleep has been an active area of SIDS research, although it has not been possible to identify infants who will die of SIDS based on respiratory patterns recorded during sleep. Also, some researchers believe that colds may contribute to SIDS, possibly by increasing stress or causing “sleep disorganization”.

Gould did not find a change in the sleep state organization in the infants with cold symptoms. Gould expected to find an increase in apnea, but instead found less apnea in the infants with mild colds during REM sleep. The decreased apnea was age-related, being most pronounced at 40 weeks and disappearing by 52 weeks. Gould suggests that the decrease of apnea in response to a cold may be a protective mechanism in normal infants.¹³

Conclusion

From the studies reviewed in this article, it appears that twins are about twice as likely to die from SIDS as singletons at a rate, and the most recent data shows that the SIDS rate for twins in the US from 1997-2000 was 1.16 SIDS deaths per 1000 live births.¹ Since a twin pair has 2 babies, this means that approximately 1 in 430 families in which twins are born will experience a SIDS death. From Malloy’s (1999) analysis, about 0.98 % of the families affected by SIDS will experience the death of both twins to SIDS. This means that approximately 1 in 48,000 families in which twins are born will lose both babies to SIDS.

Even though a baby resulting from a multiple birth may have factors that increase his or her risk for SIDS, at this time it is not possible to predict which babies will die, and most of the babies will live. The majority of babies who die of SIDS appear to be perfectly healthy and do not have significant risk factors which would make anyone suspect that they are going to become SIDS victims.

This article was prepared by a SIDS parent, and was reviewed by and distributed through the Center For Loss in Multiple Birth, Inc. Our intent is to give an overview of the current information on SIDS in multiples available in the medical literature. Although we are not medical professionals, we hope that the
material in this article is informative for SIDS families and others interested in the topic, and have made every effort to ensure that it is accurate. While the risk for triplets and higher-order multiples is undoubtedly higher than for twins, most of the available literature focuses on twins; if more information about SIDS in higher-order multiples is published in the future we will update this article. We hope that researchers will continue to study the association of SIDS and multiple births in hopes that it will shed some light on the cause of SIDS for all babies.

Alice P. Check
(Beaverton, OR; April 2003)
In honor of Patrick Ryan Check
April 26, 1987 – August 4, 1987

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