Changes in Maternal Age and the Incidence of Down Syndrome

Oh, Chang Weon; Lim, Sung Su; Kim, Ki Bok; Kee, Won Jin

Abstract

PURPOSE
The incidence of Down syndrome, the most common chromosoaml anomaly, increases with the advanced maternal age. Recently, however, the incidence of Down symdrome was reported to have decreased with wide acceptance of prenatal diagnosis and planned parenthood, prompting us to re-evaluate the incidence of Down syndrome in relation to changes in maternal age.
METHODS
Subjected to study were 296 Down cases: 26 newborn Down's among the 58,479 deliveries undergone in our hospital (KCH) during the period over 2 decades from April 1974 through December 1994; and 270 cases, whose maternal age at delivery could be traced with certainty, among 366 Down cases confirmed by genetic counseling. The observation period was divided into three: Period I, before 1979 (n=66); Period II, from 1980 to 1989 (n=127); and Period III, after 1990 (n=103).
RESULTS
1) The change in maternal age at delivery Of total 58,479 deliveries, 0.5% were born to mothers younger than 20 years, 21.3% were aged 20-25 years; 57.7% (33,722) were aged 26-30 years; 16.9% fell between 30-35 years; and 3.6% were above 35 years. Deliveries of old mothers (>35 ys) were 4.9% in Period I, which significantly decreased to 2.8% in Period II, increased again to 4.1% in Period III. 2) Changes in Down incidence among those delivered in KCH Overall Down incidence among those delivered in KCH was 0.44/1000 (26/58,479); however, it was 0.93/1000 in Period I, 0.16 in II, significantly lower; and 0.59/1000 in Period III. Of the 26 Downs born in KCH 18 were born to mothers younger than 35 years (0.03%), whereas 8 were born to mothers older than 35 years (0.8%). The Down incidence among mothers younger than 35 years did not significantly differ among the periods, while for the mothers aged over 35 years it decreased from 1.0% (6/576) in Period I to 0% (0/825) in Period II, and back to 0.3% (2/698) in Period III, indicating that Down incidence among old mothers significantly decreased recently, compared with the early period. 3) Changes in maternal age and Down incidence among those confirmed by genetic counseling. Of those 270 Down cases, 226 were born to mothers younger than 35 years, whereas 44 were delivered by old mothers aged above 36 years. Old mothers had Down babies in 23.6% (13/55) in Period I, but it significantly decreased to 11.5% (14/122) in Period II, but it recovered to 18.2% (17/93) in Period III. 4) Relationship between karyotype pattern and maternal age Of total 296 Down cases 260 (87.8%) had typical pattern of 21-trisomy, 32 (10.8%) translocation, 4 (1.4%) mosaicism. Those with typical trisomy had mean age (+/-S.D.) of 30.3 (+/-6.0) year and those with mosaicism 30.8 (+/-6.2) year, whereas those with translocation were significantly younger with the mean age of 26.6 (+/-2.5) year.
CONCLUSIONS
During the last 2 decades, the deliveries by old mothers tended to decrease, and also the overall incidence of Down syndrome has significantly decreased, especially among those born to mothers older than 35 years. Recently, however, maternal age tends to increase for various reasons, necessitating due emphasis on prenatal diagnosis.