Parity and Down's syndrome

Predictors of trisomy 21 in the offspring of older and younger women

BACKGROUND

Advanced maternal age is the only well-established risk factor for trisomy 21, yet the majority of affected individuals are born to younger women. To identify factors associated with the risk of trisomy 21 in the offspring of younger and older women, we analyzed data for cases with trisomy 21 from the Texas Birth Defects Registry for 1999 to 2007.

METHODS

Data were analyzed separately for younger (i.e., <35 years of age at delivery; n = 2306) and older (i.e., ≥ 35 years of age at delivery; n = 1811) women using Poisson regression.

RESULTS

After adjustment for maternal age and several other covariates, the prevalence of trisomy 21 in the offspring of women in both maternal age groups was higher in male than in female infants and in offspring of women who were Hispanic (compared with non-Hispanic white women) or who had at least one previous liveborn child compared to those with none. In the offspring of older women only, the prevalence of trisomy 21 was also significantly higher when the father was 20 to 24 years old (compared with 25 to 29 years old; adjusted prevalence ratio [aPR], 2.27; 95% confidence interval [CI], 1.47-3.49) and Hispanic (compared with non-Hispanic white; aPR, 1.34; 95% CI, 1.13-1.58) and among women with less than a high school education (compared with greater than high school).

CONCLUSIONS

This study identified several factors, in addition to maternal age, that were associated with trisomy 21 risk. In general, these factors were similar for both maternal age groups, although paternal characteristics were significantly associated with risk of trisomy 21 only in offspring of older women.

Maternal reproductive history and the risk of isolated congenital malformations

We examined the relationship between maternal reproductive history and the newborn's risk of isolated congenital malformations in a large case-control cohort from the Polish Registry of Congenital Malformations. Congenital malformations were classified into four categories: isolated congenital heart defects (n=1673), isolated cleft palate (n=255), cleft lip with or without cleft palate (n=448) and renal agenesis (n=103). The case groups were compared with a shared group of 2068 controls recruited in the same time period and geographic area. Multivariable logistic regression was used to assess the risk associated with maternal gravidity and of previous miscarriages after accounting for maternal age and other potential risk factors. In unadjusted analyses, maternal gravidity was significantly associated with increased risk of all four classes of congenital malformations. After adjustment, a significant association persisted for congenital heart defects [odds ratio (OR)=1.22, [95% confidence interval (CI) 1.09, 1.36], P=0.0007] and cleft lip with or without cleft palate (OR=1.21, [95% CI 1.09, 1.36], P=0.0005). A similar trend existed for isolated cleft palate (OR=1.18, [95% CI 1.02, 1.37], P=0.03). There was no appreciable increase in the risk of congenital malformations associated with a maternal history of miscarriages, but a trend for a protective effect on the occurrence of cleft lip with or without cleft palate was observed (OR=0.72, [95% CI 0.52, 0.99], P=0.045). Based on our data, maternal gravidity represents a significant risk factor for congenital heart defects and cleft lip with or without cleft palate in the newborn infant. Our data do not support an increase in risk because of past history of miscarriages.

The role of maternal reproductive history in the aetiology of neural tube defects

The contribution of environmental factors to the aetiology of neural tube defects (NTD) has been stressed over recent years and many different risk factors have been proposed. We evaluated the reproductive history of 113 NTD cases to investigate the possible role of maternal age, gestational age, sex, parity and previous pregnancy. Our results show that parity and previous spontaneous abortion can be considered as risk factors for NTD and that an accurate evaluation of reproductive history can be useful for genetic counselling. The known benefits of periconceptional intervention in reducing the incidence of NTD in high risk populations, together with our results, means that mothers with a positive reproductive history for spontaneous abortion and for multiparity are ideal subjects for folic acid periconceptional management.

Down's syndrome and maternal smoking in early pregnancy

Several previous studies have suggested that maternal smoking is associated with a decreased incidence of trisomy 21. By using the Swedish health registries, 1,321 infants with Down's syndrome (DS) were selected among 1,117,021 infants born in 1983-1993 with known smoking exposure in early pregnancy. No association between maternal smoking and all cases of DS was found [age-adjusted odds ratio (OR) for maternal smoking: 0.98; 95% confidence interval (CI): 0.86-1.11], but heterogeneity over strata existed. A slightly decreased OR (0.91; 95% CI: 0.72-1.15) for any maternal smoking was indicated among primiparas, but among multiparas, no effect of smoking on the incidence of DS could be detected (OR: 1.01; 95% CI: 0.87-1.17). The difference between the OR for smoking > or = 10 cigarettes per day among primiparas (OR: 0.59; 95% CI: 0.38-0.90) and multiparas (OR: 1.06; 95% CI: 0.86-1.31) was statistically significant. If not due to statistical fluctuation, the findings indicate that no direct effect of smoking on DS risk exists but the association observed in primiparas is due to covarying factors.

Effect of parity on human chorionic gonadotrophin levels and Down's syndrome screening

OBJECTIVE

To investigate further the association between parity and human chorionic gonadotrophin (hCG) levels during the second trimester and determine whether prenatal screening for Down's syndrome might be made more efficient by adjusting for this variable.

METHODS

Measurements of hCG were analysed in relation to parity from a cohort of 16,675 singleton pregnancies with viable outcomes, and from 108 singleton pregnancies affected by Down's syndrome. Cigarette smoking during pregnancy and maternal age were also analysed in relation to hCG levels.

RESULTS

For both unaffected pregnancies and pregnancies with Down's syndrome the median hCG levels were 7% lower in parous than in nulliparous women. A dose-response relation was also documented for the unaffected pregnancies. When para 0, para 1, and para 2 women were studied separately, hCG levels rose steadily as age advanced. Adjusting hCG levels for parity led to a 0.1% reduction in variance of the distribution of hCG measurements in unaffected pregnancies and to a negligible increase in Down's syndrome screening efficiency.

CONCLUSION

The effect of parity is not sufficiently great to warrant routine adjustment of hCG levels as part of prenatal screening for Down's syndrome.