Соотношение между последовательностью рождения близнецов и несоответствием их массы тела

¹Introduction. The concept that first-born twins are usually heavier than their second-born siblings at birth is widely believed. Various studies aimed at assessing the truthfulness of this concept have produced conflicting conclusions, both in developed and developing countries. For instance, in Nigeria, Ilesanmi et al [1] reported that among 212 twin pairs the first-born twin was heavier in 42.6% of cases while the second-born twin was heavier in 42.2% of cases. In another Nigerian study, Swende and Hwande [2] reported that the first-born was heavier in 48.7% of cases while the second-born twin was heavier in 43.6% of cases. Similarly, a study from Germany reported that mean birthweight was higher in first-born twins than in their second-born siblings [3]. Another German study involving 177 live-born twin pairs reported that the first twin was on the average heavier by 54g [4].

In contrast, a Nigerian study reported that second- born twins had weight advantage more often than their

Corresponding author: Alphonsus N. Onyiriuka.

corresponding first-born siblings [5]. Friedman et al [6] in USA, reported that second-born twins were heavier than first-born twins in 55% of cases. Similar finding has been reported in another study in a developed country [7]. Oyawoye and Fakeye [8] reported that in low birthweight twin gestations, the second-born twin was heavier than the first-born twin in 57.3% of cases. However, in these studies, the investigators did not control for gender which is well known to influence birthweight both in singletons [9, 10] and in twins [11].

Although the phenomenon of birthweight discordance is common in twin gestations, various studies on birthweight discordance were silent on which one of the discordant pair is likely to be delivered first [12-14]. This is an issue of practical clinical importance because it has been shown that twin infants with birthweightdiscordance [13-15] and second-born twins [16-18] are both at increased risk of adverse perinatal outcome. Some investigators have reported that the threshold for clinically significant birthweight discordance is 15% or more [12].

The present study sought to assess the truthfulness of the concept that first-born twins are usually heavier than their second-born siblings at birth.

Methods. This cross-sectional study involved all twin babies delivered at St Philomena Catholic Hospital (SPCH) between 1^st January, 2000 and 31^st December, 2003.

All twin babies delivered at SPCH during the 4-year study period were weighed naked by a trained midwife, within the first 30 minutes after birth, using a mechanical Waymaster weighing scale calibrated to the nearest 50 grammes. The scale was periodically standardized with known weights for reliability and daily for zero error to ensure accuracy. The order of delivery of the twins and their sexes were carefully recorded. Excluded from the study were twin babies delivered by Caesarean section and twin pairs where one or both were still-born. Only live-born pairs of twins were studied. In this study, Twin I refers to the first-born while Twin II refers to the second-born twin. The data on birth weight were analysed according to birth order to determine their relative distribution collectively. Also the difference in birth weights (weight of Twin I minus weight of Twin II) were examined to determine the magnitude and direction of such differences.

In this study, a twin pair was designated discordant if one of the pair was smaller by 15% or more. The level of discordance was calculated for each pair as a percentage of the birthweight of the heavier twin:

Level of discordance (%) = Birthweight difference×100 Birthweight of heavier twin

Intra-pair birthweight percentage differences was stratified into the following categories: 15-24.9, 2534.9 and 35% or more, which roughly corresponded to moderate, severe and extreme discordance. A percentage difference in birthweight of 14.9% and below was regarded as mild level of discordance. A twin pair was called discordant-first when the smaller was firstborn and discordant-second when the smaller was second-born. One-minute Apgar Score of 3 and below was designated as severe birth asphyxia.

The Student’s t test was used in ascertaining the level of significance of two differences, which was set at p<0.05.

Results. During the 4-year study period, a total of 104 (2.3%) sets of live-born twins were delivered (not by caesarean section) in an obstetric population of 4,544. Overall male-to-female ratio was 0.98:1.

Ratio of male Twin I to female Тwin I was 1:1. The same ratio was obtained for male Twin II and female twin II. As shown in Table 1, Twin I was more frequently heavier than Twin II (61.5% versus 28.9%). Overall, the mean birthweight of male Twin I was greater than that of male Twin II by an average difference of 83g (Table 2) p>0.05. Table 3 compared mean birthweight of female Twin I with that of female Twin II. Overall, there was no statistically significant difference in birthweight distribution. As shown in Tables 2 and 3, there was a trend towards higher mean birthweight of twin babies over the years of study, even after controlling for gender. As shown in Table 4, when both sexes were combined, Twin I was heavier by an average of 34g. The mean birthweight of male Twin I was greater than that of female Twin I by 189g (t statistic=0.862; p>0.05). Similarly, the mean birthweight of male Twin II was greater than that of female Twin II by 107g (t statistic=0.444; p>0.05) (Tables 2, 3).

Among the 30 birthweight discordant twins pairs, the mean birthweight of Twin I was 1973±423g (95% confidence interval CI=1753–2193) while the mean birthweight of Twin II was 2312±480g (95% CI=2090– 2534)±2.123 p>0.05. Of the 30 birthweight discordant twin pairs, 14(46.7%) were discordant-first and 16 (53.3%) were discordant-second. Table 5 showed twin birth order in relation to levels of birthweight discordance. Each of the three sets of twins in which both foetuses were stillborn exhibited severe levels of birthweight discordance greater than 25% (specifically, their levels of birthweight discordance were 26.7, 36.6 and 60% in first, second and third twin pairs respectively ). As shown in Table 6, Twin I is more likely to be heavier than Twin II when the birthweight difference between the pair is between 500-750 g. When the birthweight difference exceeds 750 g, the probability that Twin I will be heavier than Twin II is 83.3% (5 of 6). The risk of breech delivery and 1-minute Apgar Score of 3 and below were each 1.8 times higher in Twin II than in Twin I (Table 7).

Discussion. Data from the present study showed that the widely held view that first-born twins were usually heavier than their second-born siblings was incorrect. The differences in mean birthweights between the first-born and the second-born twins were small and statistically not significant, making it less likely to be important clinically. However, this does not imply that significant clinical problem could never occur.

In the present study, excluding twins of equal birthweight, first-born twins had weight advantage more often than their second-born siblings at birth. Other previous studies [1, 2, 19] in Nigeria have reported similar trend but with differing percentages. In contrast, some other investigators [5-7] have reported that second-born twins were more often heavier than their first-born siblings. There is no readily available explanation for this finding. It is possible that it is an integral part of the discordant phenomenon. This view is reinforced by the finding in this study that the probability that Twin I will be heavier that Twin II was 83.3% when the birthweight difference between the twin pair was greater than 750 g.

Overall, comparing the mean birthweight of male twins I and II, the former was heavier than the latter by an average of 83 g. However, this difference was not statistically significant. On the other hand, similar comparison between female twins I and II, showed that both had similar birthweight distribution with a difference of only one gramme. Comparison with previous studies [1-3, 5, 6] was not possible because the authors did not control for gender in their analysis of relative birthweights of Twins I and II. In both singletons [10] and twins [11], it has consistently been shown that males were significantly heavier than females at birth. It is possible that failure to control for gender in relative birthweight data analysis may have partly contributed to the conflicting reports in literature concerning relative birthweights in twins. Consequently, it is suggested that future studies aimed at verifying the truthfulness of the concept that first-born twins were usually heavier than second-born twins should take gender into consideration in their data analysis.

Conclusion. The results of the present study indicated that second-born twins demonstrated a higher tendency toward low birth weight, breech presentation and low Apgar Score compared to their first-born counterparts. Similar finding has been reported by other investigators [3, 8, 17, 20]. These three factors above may account for the higher perinatal death rate in second-born twins compared to their first-born counterparts.

Table 1

Relative birthweights of Twins I and II

Relative birthweights	No of babies	Percentage
Twin I greater than Twin II	64	61.5
Twin I less than Twin II	30	28.9
Twin I equal to Twin II	10	9.6
Total	104	100

Table 2

Comparison of mean birthweight of male Twin I and male Twin II.

Year of	Mean Birthweight in grammes
Study	Twin I	Twin II	*Difference	t-statistic	P value
2000	2175±402	2297±411	-122	0.832	>0.05
2001	2430±417	2348±420	82	0.493	>0.05
2002	2638±503	2422±427	216	1.309	>0.05
2003	2925±518	2657±513	268	1.272	>0.05
Total	2515±427	2432±435	83	1.028	>0.05

* Difference = mean birthweight Twin I minus mean birthweight of Twin II.

Table 3

Comparison of mean birthweight of female Twin I and female Twin II

Year of	Mean Birthweight in grammes
Study	Twin I	Twin II	*Difference	t-statistic	P value
2000	2154±479	2157±481	-3	0.019	>0.05
2001	2150±493	2175±487	-25	0.123	>0.05
2002	2388±487	2642±553	-254	1.243	>0.05
2003	2609±523	2005±517	204	1.003	>0.05
Total	2326±445	2325±501	1	0.011	>0.05
	Comparison of mean birthweight of Twin I and Twin II (both sexes combined)				Table 4
Year of	Mean Birthweight in grammes
Study	Twin I	Twin II	Difference	t-statistic	P value
2000	2162±382	2225±403	-63	0.671	>0.05
2001	2318±445	2251±479	67	0.512	>0.05
2002	1878±332	1874±359	4	0.011	>0.05
2003	2731±603	2550±586	181	1.159	>0.05
Total	2416±467	2380±472	36	0.276	>0.05

Birth order in 30 birthweight-discordant twin pairs

Table 5

Relative birthweights of Twins I and II	Levels of birthweigt discordane in percentage
	15-24.9 (moderate) No (%)	25-34.9 (severe) No (%)	35 or more (Extreme) No (%)	Total No (%)
Twin I greater than Twin II	11 (36.7)	4 (13.3)	1 (3.3)	16 (53.3)
Twin I less than Twin II	8 (26.7)	4 (13.3)	2 (6.7)	14 (46.7)

Table 6

Distribution of birthweights differences according to relative birth weight in 30 birthweight-discordant twin pairs

Relative birthweights of Twins I and II	Birthweight difference (g) in twin pairs
	<500g No (%)	500-750g No (%)	>750g No (%)
Twin I greater than Twin II	2 (6.6)	11 (36.7)	5 (16.7)
Twin I less than Twin II	6 (20.0)	5 (16.7)	1 (3.3)
Twins I and II combined	8 (26.6)	16 (53.4)	6 (20.0)

Table 7

A. Low birth weight n=119	No of babies	Percentage
Twin I less than 2500g	55	46.2
Twin II less than 2500g	64	53.8
B. Twin birth order and presentation
Twin I: n=104
-73        Cephalic	76	73.1
-74        Breech	25	24.0
-75        Transverse	3	2.9
Twin II: n=104
-76        Cephalic	59	55.7
-77        Breech	44	42.3
-78        Transverse	1	0.1
C. One-minute Apgar Score=or<3 n=14
Twin I	5	35.7
Twin II	9	64.3
D. Perinatal death n=8
Twin I	2	25.0
Twin II	6	75.0

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