with the introduction of ovulation - inducing agents in the late 1960s and assisted - reproductive techniques in the 1970s , the frequency of multiple pregnancies and multiple birth have been increasing ( 1 - 5 ) . in korea ,
the frequency of twins has recently been increased by three times for the past 20 yr ( 6 ) . to date , however , no studies have been conducted to examine the triplets . in cases of multiples , as compared with singletons , the morbidity and mortality of fetus or newborns were relatively high .
this has therefore been of increasing interest ( 5 - 8 ) . particularly in cases of multiples with the intrauterine growth retardation , the morbidity and mortality
have been reported to be relatively high ( 9 , 10 ) . according to blickstein ( 9 , 10 )
the author therefore noted that the differential criteria for birth weight depending on the gestational age should be established .
in addition , criteria for birth weight depending on the gestational age vary depending on country , ethnicity and sex . accordingly , the differential criteria for birth weight depending on the gestational age should be established . in korea ,
we conducted this study to establish the normal value of birth weight depending on the gestational age in korean triplets .
of the data about the population status , which was collected by the korean national statistics office , we used a raw data about birth records of a 10-yr period from january 1 1998 to december 31 2007 .
of these , excluding 20,519 newborns ( 0.4% ) with unknown gestational age , birth weight or plurality , 840 triplets , aged between gestational weeks 27 and 38 , were finally enrolled in the current study .
the number of triplets , whose gestational age was shorter than 27 weeks , as present at an extremely small .
triplets whose gestational age was longer than 38 weeks were also not enrolled in the current study .
we obtained mean birth weight , standard deviation and 10th , 25th , 50th , 75th , and 90th percentile values for each gestational age group by one week increment .
then , we investigated the birth weight distribution of each gestational age group by the normal gaussian model . to establish final standard value of birth weight distribution by gestational age
, we used the finite mixture model to eliminate erroneous birth weights for respective gestational age ( 15 - 18 ) .
a baseline value of birth weight depending on the gestational age in triplets , which was established as described herein , was compared with 10th , 50th , and 90th percentile values in singletons and twins ( 12 ) .
a comparison was also made with criteria for triplets which have been established in other countries ( 19 , 20 ) .
, college station , tx , usa ) for an analysis and an estimation of finite mixture model .
during a 10-yr period from 1998 to 2007 , the total number of newborns who were born between gestational weeks 27 and 38 was 1,330,822 . of these ,
the number of singletons , twins and triplets was 1,330,822 , 90,245 , and 840 , respectively .
a mean gestational age was 37.31.5 weeks , 36.02.0 weeks , and 33.32.4 weeks in the corresponding order .
a mean birth weight was 3,071490 g , 2,414455 g , and 1,836454 g in the corresponding order .
these results indicate that gestational age and birth weight decreased depending on the plurality ( table 1 ) .
mean age of mothers was 29.7 yr in cases of singletons , 30.2 yr in cases of twins and 30.5 yr in cases of triplets .
these results indicate that age of mother increased significantly depending on the plurality ( table 1 ) .
the birth rate of triplets was increased from 0.06% in 1998 to 0.1% in 2007 .
a mean gestational age of male triplets was 33.32.6 weeks and that of female triplets was 33.32.3 weeks .
however , mean birth weight of male babies was 1,872495 g and that of female babies was 1,799407 g. this difference reached a statistical significance ( p=0.001 ) .
in addition , birth weight percentile for gestational age was compared between male and female triplets .
this showed that male triplets had a significantly greater birth weight percentile for the gestational age as compared with female triplets in overall gestational age ( table 3 ) .
birth weight percentile for gestational age in triplets was compared with the normal value which we obtained using the same methods in singletons and twins in our previous study ( 12 ) . according to this , in the overall gestational age ranging from gestational week 27 to 38
, it was relatively smaller as compared with birth weight obtained from twins and singletons ( table 4 , fig .
we compared 50th percentile curve of korean triplets with those of usa and norway , it was smaller than those in the usa and norway ( fig .
in korea , the birth rate has been annually decreasing . however , with the advancement of assisted reproductive techniques such as ovulation - inducing agents ,
the birth of multiples has been increased . with the well - trained neonatal intensive care unit personnel treating newborns and the well - equipped facility ,
the survival rate of premature birth has also been increased ( 6 , 21 ) .
they also are associated with such problems as premature birth , intrauterine growth retardation and low birth weight ( 5 - 8 ) .
although multiples have been of increasing interest , few studies have been conducted to examine triplets in korea .
the current study was conducted using a raw data about birth records which was collected during the recent 10-yr period by the korean national statistics office . and
it would be of significance in that it first obtained the normal value of birth weight depending on the changes in the plurality of triplets , mean gestational age , mean birth weight , mean age of mothers and gestational age . according to a review of english literature ,
triplets have been abruptly increased during the recent 20- to 30-yr period ( 1 , 3 - 5 ) . according to the current study , however , it was shown that triplets were increased by approximately two times during a 10-yr period from 1998 to 2007 in korea .
other studies have shown that the birth rate of multiples was increased as mothers ' age was increased ( 1 , 4 ) . in korea , it has also been shown that mean age of mothers of singletons , twins and triplets was increased .
these reports suggest that the increased birth rate of triplets originated from the increased age of mothers and the development of assisted reproduction technology . in the present study ,
birth weight was significantly greater in male triplets when compared with female triplets at the same gestational age .
this implies that an intrauterine growth pattern may vary genetically between male and female triplets .
triplets had a lower birth weight as compared with singletons or twins , which was also in agreement with the previous reports ( 22 , 24 - 26 ) in particular , from gestational week 32 on , there was a great different from singletons ( table 3 ) . in regard to this , other authors noted that no growth acceleration occurred during the third trimester of pregnancy due to the limitation of intrauterine space ( 9 , 10 , 22 , 27 ) .
other authors noted , however , that there was no significant difference in the birth weight between singletons , twins and triplets prior to the third trimester of pregnancy . according to the present study ,
however , there was a significant difference in mean birth weight from gestational week 27 .
when 50th percentile of birth weight of korean triplets was compared with those of usa and norway ( 19 , 20 ) , it was smaller than those in the usa and norway during a period from gestational week 28 to 36 .
. it might be due to an insufficient amount of the data , however , that the normal value was relatively greater during a period ranging from gestational week 37 to 38 in korea . in cases of triplets , however , the proportion of cases in which a delivery occurred at gestational week 36 or later was approximately 10% ( 28 ) .
limitations of the current study are as follows : 1 ) in a statistical analysis of the birth records which were collected during a 10-yr period in korea , the number of triplets aged gestational week 27 or earlier was extremely small .
2 ) the data which was collected from triplets aged gestational week 37 or later was also problematic . despite these limitations ,
the current results are of significance in that it first obtained the normal value of birth weight during a period ranging from gestational week 27 to 38 , which is clinically important , in korean triplets .
it is expected that the current results would be of help for the studies and treatment of triplets .
 an estimation of the baseline value of birth weight depending on gestational age is helpful for reducing morbidity and mortality following the early diagnosis and treatment of intrauterine growth retardation . in korea , 
 there are established baseline values for singletons and twins . 
 but no definite criteria exist for triplets yet . 
 given the above background , we obtained the baseline value of birth weight depending on the gestational age in triplets with a gestational age of 27 - 38 weeks using a raw data about birth records which had been obtained during a 10-yr period from 1998 to 2007 . 
 this baseline value was compared with those of singletons and twins . during the 10-yr period , 
 the total number of newborns who were born between gestational age 27 and 38 was 1,330,822 . 
 of these , the number of singletons , twins and triplets was 1,330,822 , 90,245 , and 840 , respectively . 
 a mean gestational age was 37.31.5 weeks , 36.02.0 weeks and 33.32.4 weeks in the corresponding order . 
 a mean birth weight was 3,071490 g , 2,414455 g , and 1,836454 g in the corresponding order . 
 a comparison of the birth weight depending on the gestational age of triplets was made with the normal value of singletons and twins . according to this , in the overall gestational age ranging from weeks 27 to 38 
 , it was relatively smaller as compared with the birth weight of twins and singletons . 
 the current study was of significance in that it first obtained the normal value of birth weight of triplets in the overall gestational age ranging from weeks 27 to 38 , whose results are expected to be helpful for studies or treatments of triplets . 