Clinical Genetics 1986: 2 9 384-388

The effect of consanguineous marriages on reproductive wastage

S. A. AL-AWADII, K. K. NAGUIB~, M. A. MOUSSA~, T. I. FA RAG^, A. S. BEBI~ AND M. Y. EL- KHALIFA'

'Kuwait Medical Genetics Centre and 2Department of Community Medicine, Faculty of Medicine, Kuwait University, Kuwait

A stratified representative sample size of 5,007 Kuwaiti females aged 15 years and above was drawn during 1983 and structurally interviewed to study the influence of consanguineous marriages (up to the second cousin) on reproductive wastage. Losses comprised prenatal deaths (abortions and stillbirths) and neonatal deaths (up to the first month of life). The rate of consanguineous mating in the sample was 54.3% with 95% confidence limits estimated rate 52.9% to 55.7% when projected over the whole Kuwaiti population. First cousin marriages accounted for 30.2% of the sample followed by 22.1 YO less than first cousin (first cousin once removed and second cousins) and 2% only double first cousin. The study showed higher prenatal and neonatal losses among consanguineous (l4.2%, 2.97%) than nonconsanguineous (13.97%, 2.54%) although not statistically significant. No consistent increase in reproductive wastage was evident as the inbreeding coefficient, F, advances mainly because of decline in the wastage rate among the double first cousin marriages which represents only 2% of our sample.

Received I 1 March 1985, revised, accepted for publication 3 March 1986

Key words: Consanguinity; reproductive wastage.

Several authors have studied the effect of consanguinity on reproductive wastage in human populations (Sutter & Tdbah 1952a, Bemiss 1958, Schull & Neel 1965, El-Alfi et al. 1968, 1969, Schull & Neel 1972, Schull et al. 1970, Rao & Inbaraj 1980). Some of these studies showed significant inbreeding effects while others did not.

Generalisation of the outcome of these studies to other populations may be invalid since each study reflects the genetic charac- teristics, which are governed by socio-cul- tural ethnic, religious and economic factors of the concerned population. Indeed, the validity of comparing the results of different studies may be questionable because of the variation in approach, methods and repre-

sentativeness of the studied cross-section to its population. In view of these facts we decided to investigate the rate as well as the influence of consanguineous marriages on reproductive losses in our highly inbred population. Losses comprised prenatal deaths (abortion & stillbirth) and neonatal deaths (up to the first month of life).

Material and Methods

A representative sample of 5,007 Kuwaiti females, aged 15 years and above, was se- lected during 1983. The size was found ad- equate to achieve a high degree of precision in estimating the population inbreeding rate. The sampling plan was stratified

t

C O N S A N G U I N I T Y A N D R E P R O D U C T I V E W A S T A G E 385

(based upon geographical location) with proportional allocation according to stra- tum size (Cochran 1977).

In Kuwait there are 22 antenatal & gynae- cological clinics uniformly distributed all over the country. There is no formal clear- cut classification for socio-economic levels since the inhabited part of the country is very limited with no distinct climatic, eco- nomic or socio-cultural variation. Besides, the socio-economic status is the resultant of many factors like education, housing, in- come, occupation, nutritional and rearing habits. Because of the smallness of the population (126,920 females 1549 years old, the child-bearing age) and the mixed nature of the community, the socio-econ- omic status can be hardly defined. Ante- natal and gynaecological clinics in all Ku- wait districts representing all socio-econ- omic classes were involved. The number required in each clinic was predetermined to be proportional to the size of the district. A standard format was designed for the purpose, including information about de- gree of consanguinity, between the two spouses if the subject was married & about the parents if unmarried, number of live- borns, abortions, stillbirths and neonatal deaths. Gynaecologists were instructed to structurally interview and complete a for- mat for a consecutive series of Kuwaiti fe- males 15 years and above attending their clinics until the required number was reach- ed in each stratum. Standard statistical methods were used to analyse the data: The chi-squared test was used to assess the as- sociation between two variables; and the Z normal test was used to show the difference between two proportions. The relations be- tween the coefficient of inbreeding (F), pre- natal (abortions and stillbirths) and neo- natal (up to the first month of life) losses were studied using MCM theory (Morton et al. 1956). This theory assumes an ex- ponential relationship.

M = ~ L c ( * + ~ ~ ) ’ , where M is the wastage rate, A is the estimate of damage in a non-inbred population (F=O) and B is the regression coefficient, an estimate of the number of deleterious equivalents per gamete (le- thons).

The wastage rate, M has been considered for abortions on the basis of the number of pregnancies; for stillbirths on the basis of born children; for neonatal deaths on the basis of children born alive. The prenatal wastage rate comprised relating the sum of abortions and stillbirths to the number of pregnancies. The total wastage rate was ob- tained by relating the sum of abortions, stillbirths and neonatal deaths to the num- ber of pregnancies.

Results

The rate of consanguineous marriages in the sample was found to be 54.3% with 95% estimated population confidence limits 52.9 to 55.7%. Table 1 shows the distri- bution of the sample according to the degree of consanguinity.

Table 2 depicts the prenatal losses classi- fied according to the degree of parental con- sanguinity. The rate of abortion among the first cousin marriages (12.44%) was higher than that among the non-consanguineous group (11.7%). This higher rate, although not statistically significant, was reflected on

Table 1

Distribution of 5,007 Kuwaiti females according to the degree of consanguinity

Degree %

Double first cousins 2.0 First cousins 30.2 Less than first cousins (First cousins once removed and second cou- sins) 22.1

Total consanguineous 54.3

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Table 2

The prenatal deaths classified according to the degree of parental consanguinity

Preg- Total nancies Children Still- prenatal

Degree of consanguinity F (n) Abortions (%) born (n) births (YO) losses (YO)

Nonconsanguineous 0 6486 759 (11.70) 5727 147 (2.57) 13.97 Less than first cousins 0.0234 3594 415 (11.55) 3179 86 (2.71) 13.94 First cousins 0.0625 4824 600 (12.44) 4224 101 (2.39) 14.53 Double first cousins 0.1250 259 30 (11.58) 229 2 (0.87) 12.36 Total consanguineous 0.0219 8677 1045 (12.04) 7632 189 (2.48) 14.22

Table 3

The neonatal and total deaths classified according to the F value

Neonatal Total F Children born alive deaths % wastage Yo

_______ ~

0 5580 142 (2.54) 1048 (16.16) 0.0234 3093 84 (2.72) 585 (16.28) 0.0625 4123 134 (3.25) 835 (17.31) 0.1250 227 3 (1.32) 35 (13.51) 0.0219 7443 221 (2.97) 1455 (16.77)

the total consanguineous abortion rate (12.04%). On the contrary, the stillbirth rate did not show any trend as F changes. The total prenatal losses showed a consistent increase as F increases; however there was decline in the double first cousins category. The prenatal losses were in general higher among the consanguineous group (14.22%) than among the non-consanguineous

Table 3 shows that the rate of both neo- natal deaths and total wastage increases as F increases except at the double first cousins category. The neonatal deaths and total wastage were higher among the consanguin- eous (2.97% and 16.77%, respectively) than among the non-consanguineous (2.54% and 16.16%, respectively). However, these in- creases were not statistically significant.

The relation between the coefficient of inbreeding F, and abortion showed that the mean number of lethal equivalents per ga- mete (lethons) as estimated according to the

(1 3.97%).

MCM formula was 0.00410.06 (A=0.126). These estimates were obtained using the four values of F shown in Table 2. However, when the same procedure was followed to hold a relation between stillbirths and F, there was a negative relationship (A = 0.029, B = -0.146+0.04); this was caused by the drop in the double first cousin stillbirth rate (0.87%). The same applied to the total pre- natal losses (A=0.155, B = -0.141 +_0.09), neonatal deaths (A = 0.03, B =

- 0.098 f 0.08) and total wastage (A = 0.185, B = - 0.241 k 0.18). This inconsistent relation between F and prenatal, neonatal and total losses were caused by the decline in their rates among the double first cousin marriages (12.36%, 1.32% and 13.51%, re- spectively).

Discussion

Several studies were carried out dealing with the effect of inbreeding on reproductive

C O N S A N G U I N I T Y A N D R E P R 0 D U C T I V E W A S TAG E 387

wastage and ended up with variable infer- ences drawn from them. Some authors de- monstrated a significant role: (Sutter & Ta- bah 1952, 1953, Slatis et al. 1958, El-Sherbi- ni 1967, Stern 1973, Freire-Maia et al. 1983).

Other authors reported no effect: (Fra- ser & Biddle 1976, Rao & Inbaraj 1977, 1980, Lindelius 1980, Asha et al. 1981). No doubt, these results are not amenable to extension to our population because of the variation in population structure, genetic, socio-cultural, ethnic, religious and econ- omic factors in addition to the variations in approaches, methods and representative- ness of the studied cross-section of the population.

In the present study the rate of con- sanguineous marriages was 54.3%, which is considered rather high compared to other parts of the world. The demographic struc- ture of the Kuwaiti population is rather pe- culiar because of the rapidly developing manpower needs. Kuwaiti constitute 41.7% of the population (1,357,952)* while the ma- jority (58.3%) are non-Kuwaitis from differ- ent ethnic origins. To neutralise the effect of ethnicity we focused upon Kuwaitis only. For a long time, historically, consanguin- eous marriages were practised by different socio-economic classes among Arabs in gen- eral and Kuwaitis in particular (El-Alfi et al. 1968, Badr 1972, Hafez et al. 1983, Al- Awadi et al. 1985). Before the discovery of oil in 1934, the Kuwait population was very small and to a large extent could be con- sidered as genetically isolated; in addition to the powerful social pressure for intermar- riages, which were usually arranged at an early age by the parents. Also, the desire to keep owned properties within the family played a role in the elevation of the high incidence of intermarriages.

Our results showed a higher rate of abor-

tions, stillbirths, neonatal deaths and total wastage among the consanguineous group; although not statistically significant. This finding may be partially due to confounding of mortality data by the socio-economic variable or due to the fact that 1/4 of the total load (2 lethon plus morbons) affects abortions and stillbirths leaving, therefore, the largest part of it for postnatal events (unpublished data). Our study included only neonatal (up to the first month of life) deaths and not further postnatal deaths.

Recently, Freire-Maia (1984) found a non-significant difference of the prenatal genetic load among the consanguineous and non-consanguineous (15% 8~ 12%, respec- tively, in his studied population). While the postnatal genetic load among the first cou- sin group was double that of the non-con- sanguineous groups (16% & 8%, respec- tively).

In fact, a consanguineous marriage in- creases the risk of recessive hereditary dis- eases and a polygenic one in their children by allowing the chance of the detrimental recessive genes to become an homozygous state manifested by biochemical defects or congenital malformations. These defects may interfere with the adaptability to in- trauterine life and extrauterine environ- ment. The effect of inbreeding will be influ- enced by the duration of practicing con- sanguineous marriages (Roberts & Pembrey 1978), since long term practice reduces the burden of deleterious genes and this may explain why some studies showed positive inbreeding effects while others did not.

In conclusion, this study did not show a consistent trend in reproductive wastage as the degree of consanguinity advances. How- ever, it showed higher overall rates of differ- ent reproductive losses among the con- sanguineous than in the non-consanguin- eous groups as a whole.

*Kuwait census 1980, Part 1 , Ministry of Plan- ning, The Central Statistical Office Press, 1982.

388 A L - A W A D I E T A L

Acknowledgments

We would like to express our thanks to Mr. Mohammed Younis for his help in com- puter services and to Mrs. Sarala Ramach- andran for typing the manuscript.

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