Trisomy 21 (Down Syndrome)
Alaska, 2007-2017
Background
Down syndrome, or Trisomy 21, is a condition in which an infant is born with an extra copy of chromosome 21. Chromosomes are small packages of genetic material responsible for inherited traits. They determine how the body forms and grows during pregnancy and how it will function after birth. Typically, an infant has 23 pairs of chromosomes: half from each parent. An infant with Down syndrome has a third copy of chromosome 21, which can come from either parent.[1] Down syndrome is also referred to as Trisomy 21 because an infant with the condition has three copies of chromosome 21. This extra chromosome changes how the infant’s body and brain develops, causing intellectual and physical abnormalities. Although Down syndrome can often be identified at birth by the presence of specific physical traits, including a slightly flattened facial profile, an upward slant to the eyes, and a single line across the palm of the hand, it is sometimes identified in the first trimester of pregnancy by prenatal screening.[2] Children with Down syndrome often experience delays in speech and cognition as well as hearing and vision problems. More severe health problems can include heart defects, thyroid disease, and anemia.[3]
Most cases of Down syndrome are “spontaneous,” they occur when the mother’s eggs or father’s sperm cells are developing, for no obvious reason. It may be related to environmental or genetic factors. Certain risk factors, like mothers who are over 35 years old or families who have another child with Down syndrome, may increase the probability of having an infant with Down syndrome.[4,5]
Epidemiology
Alaska Birth Defects Registry (ABDR) registers birth defects as reported from health care providers using International Classification of Disease (ICD) billing codes. The use of these ICD codes can lead to misclassification of diagnosed conditions. Prior to this report, all prevalence estimates were based on the number of unique children reported to ABDR with an ICD code representing a specified condition regardless of case confirmation status.
The estimates in this report were derived by conducting medical record review and case confirmation of a random sample of cases of the condition reported to ABDR. The confirmation probability from the sample was used to develop informed estimates of the actual diagnosed defect prevalence. See Defect prevalence calculation.
For explanations of table columns see Column descriptions.
Prevalence
Down syndrome is the most common Trisomy, occurring in 14.3 out of every 10,000 live births in the United States. This results in about 6,000 babies diagnosed with Down syndrome nationally each year.[2]
In Alaska, during 2007-2017, the prevalence of Down syndrome was 13.9 per 10,000 live births.| Reports | Defects | Births | Prevalence (95% CI) | |
|---|---|---|---|---|
| Total | 196 | 172.4 | 123630 | 13.9 (12.0, 16.2) |
| Notes: 95% CI = 95% Confidence Interval | ||||
Trend
Prevalence per 10,000 births of Down syndrome during 2007-2017 by three-year moving averages, with 95% confidence interval band and Poisson estimated fitted line.
| Reports | Defects | Births | Prevalence (95% CI) | Predicted Prevalence† | |
|---|---|---|---|---|---|
| 2007-2009 | 17.7 | 15.5 | 11262.0 | 13.8 (8.2, 21.9) | 14.7 |
| 2008-2010 | 18.0 | 15.8 | 11404.3 | 13.9 (8.3, 22.0) | 14.4 |
| 2009-2011 | 17.0 | 15.0 | 11404.7 | 13.2 (7.8, 21.3) | 14.2 |
| 2010-2012 | 21.0 | 18.3 | 11354.0 | 16.1 (10.1, 25.1) | 13.9 |
| 2011-2013 | 21.7 | 18.8 | 11349.0 | 16.6 (10.3, 25.5) | 13.6 |
| 2012-2014 | 17.7 | 15.6 | 11334.7 | 13.7 (8.2, 21.8) | 13.4 |
| 2013-2015 | 13.7 | 12.3 | 11377.3 | 10.8 (5.9, 18.0) | 13.1 |
| 2014-2016 | 13.7 | 12.3 | 11295.0 | 10.9 (6.0, 18.1) | 12.8 |
| 2015-2017 | 17.3 | 15.2 | 10979.7 | 13.9 (8.2, 22.2) | 12.6 |
|
Notes: Each row is based on three-year moving averages; Prevalence reported per 10,000 live births; 95% CI=95% Confidence Interval † Estimated rate based on Poisson model |
|||||
| Estimate | Std. Error | t value | Pr(>|t|) |
|---|---|---|---|
| -0.01939 | 0.01877 | -1.03268 | 0.33611 |
Regional Distribution
Distribution of Down syndrome in Alaska by Public Health Region of maternal residence at the time of birth. A description of regional breakdowns can be found here. Data suppressed for # of reports < 6.
| Reports | Defects | Births | Prevalence (95% CI) | |
|---|---|---|---|---|
| Anchorage | 86 | 75.3 | 50455 | 14.9 (11.9, 18.6) |
| Gulf Coast | 12 | 10.6 | 7725 | 13.7 (7.1, 23.8) |
| Interior | 38 | 33.3 | 22487 | 14.8 (10.5, 20.6) |
| Mat-Su | 20 | 17.8 | 14937 | 11.9 (7.1, 18.2) |
| Northern | 11 | 9.8 | 8520 | 11.5 (5.6, 20.1) |
| Southeast | 7 | 6.5 | 7625 | 8.5 (3.7, 17.1) |
| Southwest | 22 | 19.2 | 11881 | 16.1 (10.3, 25) |
| Notes:Prevalence reported per 10,000 live births; Data suppressed for # of reports < 6; 95% CI = 95% Confidence Interval | ||||
Demographics
Some subgroups may be more at risk for having a baby with Down syndrome. This section provides the descriptive epidemiology of specified maternal, birth, and child characteristics identified from the birth certificate.
| Reports | Defects | Births | Prevalence (95% CI) | |
|---|---|---|---|---|
| Sex | ||||
| Female | 105 | 91.7 | 59998 | 15.3 (12.4, 18.6) |
| Male | 91 | 80.6 | 63632 | 12.7 (10.1, 15.7) |
| Birth weight (grams) | ||||
| <2500 | 42 | 35.1 | 7231 | 48.5 (34.9, 67.3) |
| 2500+ | 152 | 135.7 | 116215 | 11.7 (9.8, 13.8) |
| Maternal age | ||||
| 12-19 | 9 | 8.3 | 9156 | 9.0 (4.5, 17.2) |
| 20-24 | 33 | 30.2 | 32872 | 9.2 (6.4, 13.0) |
| 25-29 | 32 | 29.9 | 37743 | 7.9 (5.4, 11.0) |
| 30-34 | 42 | 37.1 | 28152 | 13.2 (9.6, 18.1) |
| 35-39 | 46 | 38.9 | 12614 | 30.8 (22.0, 41.4) |
| 40+ | 34 | 28.1 | 3064 | 91.8 (63.4, 132.1) |
| Maternal race | ||||
| Alaska Native/American Indian | 53 | 46.4 | 31560 | 14.7 (10.9, 19.4) |
| Asian/Pacific Islander | 25 | 21.6 | 11810 | 18.3 (11.7, 27.2) |
| Black | 10 | 8.7 | 5081 | 17.1 (8.1, 31.0) |
| White | 106 | 93.9 | 73595 | 12.8 (10.3, 15.5) |
| Maternal education (years) | ||||
| <12 | 21 | 18.3 | 11929 | 15.4 (9.6, 23.9) |
| 12 | 63 | 55.7 | 43162 | 12.9 (9.8, 16.6) |
| 12+ | 97 | 85.7 | 65105 | 13.2 (10.6, 16.1) |
| Marital status | ||||
| Married | 127 | 111.5 | 78396 | 14.2 (11.8, 17.0) |
| Unmarried | 66 | 58.3 | 44625 | 13.1 (10.1, 16.8) |
| Maternal smoking use | ||||
| Reported smoking | 28 | 24.5 | 16817 | 14.6 (9.6, 21.2) |
| Reported not smoking | 163 | 143.6 | 104707 | 13.7 (11.6, 16.1) |
| Medicaid (mother or child) | ||||
| Medicaid | 129 | 111.6 | 62666 | 17.8 (14.7, 21.3) |
| non-Medicaid | 67 | 60.7 | 60839 | 10.0 (7.7, 12.7) |
| Father on birth certificate | ||||
| None | 17 | 14.5 | 6328 | 23.0 (13.3, 37.1) |
| Present | 179 | 157.9 | 117302 | 13.5 (11.4, 15.7) |
| Notes: Prevalence reported per 10,000 live births; Data suppressed for # of reports < 6; 95% CI = 95% Confidence Interval | ||||
Technical notes
Column descriptions
# Reports: Unless otherwise noted, the number of unique reports of the defect received by ABDR during the specified birth year(s). Each report represents a unique child with the specified defect.
# Defects: The estimated true number of reports that are diagnosed defects based on medical record review and case confirmation.
# Births: The number of live births among Alaskan residents that occurred in Alaska during the specified birth year(s).
Prevalence (95% CI): The estimated diagnosed prevalence of the condition and corresponding 95% Confidence Interval. (For information on how the defect prevalence was estimated see below).
Defect prevalence calculation
The estimated defect prevalence was calculated using a Bayesian approach based on the reported prevalence, PPV and 1-NPV (see formula below).
Through medical records review and case confirmation of a random sample of reported cases, the defect prevalence is calculated as:
\[PPV (Positive Predictive Value) = p(defect|report)\] \[NPV (Negative Predictive Value) = p(\overline{defect}|\overline{report})\]
\[p(defect) \approx [p(report)\cdot PPV]+[p(\overline{report})\cdot (1-NPV)]\]
Defect prevalence estimates are a more accurate estimation of the actual diagnosed prevalance of birth defects compared to the reported prevalance estimates in Alaska. ABDR obtains reports from medical providers using International Classification of Disease (ICD) codes that are extracted from individual systems which when aggregated may not reflect true diagnostics. Caution should be used when interpreting and comparing the reported prevalence estimates with national estimates.
See Data analysis methods for more information.
P-value estimate
To evaluate the trend over time and account for under/over-dispersion we constructed a quasi-Poisson regression model. This model assumes the variance is a linear function of the mean and models the estimated number of annual defects by year with a natural log (ln) offset of the annual births. P-values < 0.05 are considered significant, which indicates that the predicted slope is significantly different from a slope of zero.
Data suppression
For region and demographic data tables, values are suppressed based on the number of reports received during the observation period. Counts less than 6 are suppressed (as indicated by ‘-’ in the table). For regions or demographics with only one cell count suppressed a second is suppressed to eliminate the ability to back-calculate the estimate.
References
[1] NIH U.S. National Library of Medicine. Genetics Home Reference: Down Syndrome, https://ghr.nlm.nih.gov/condition/down-syndrome.pdf; 2017 [accessed 02.23.2017]
[2] Witters I, Van Robays J, Willekes C et al. Trisomy 12,18, 21, Triploidy and Turner syndrome: the 5T’s. Look at the hands. Facts, Views & Vision in ObGyn 2011; 3(1): 15-21.
[3] Centers for Disease Control and Prevention, Facts about Down Syndrome, https://www.cdc.gov/ncbddd/birthdefects/downsyndrome.html; 2016 [accessed 02.23.2017]
[4] Copped F. Risk factors for Down syndrome. Archives of Toxicology 2016; 90(12): 2917-2929.
[5] Gaulden ME. Maternal age effect: the enigma of Down syndrome and other trisomic conditions. Mutation Research 1992; 296(1-2):69-88
Resources
National Down Syndrome Society
Centers for Diesase Control and Prevention
National Birth Defects Prevention Network
Suggested Citation
State of Alaska Department of Health and Social Services, Division of Public Health, Section of Women’s, Children’s, and Family Health. Alaska Birth Defects Registry Condition Report: Trisomy 21, Alaska, 2007-2017. Updated June 30, 2021. Available at: http://rpubs.com/AK_ABDR/Trisomy21_07_17.
Contact
Alaska Birth Defects Registry (ABDR)
3601 C Street, Suite 358
Anchorage, AK 99503
(907) 269-3400 phone
(907) 754-3529 fax
hssbirthdefreg@alaska.gov
Updated: June 30, 2021
Code source: R:\ABDR\Analysis_New\ABDR_CASECONF\cond_reports\Published_reports\Trisomy21_07_17.Rmd
