Background

Anencephaly is a neural tube defect (NTD) or brain disorder that results when the upper part of the neural tube does not close all the way. The neural tube is essential in forming the infant’s brain, skull, and spinal cord. The incomplete closure of the upper part of the neural tube results in a infant being born without the forebrain (the front part of the brain) and the cerebrum (the thinking and coordinating part of the brain). The remaining brain tissue is often left exposed, uncovered by bone or skin. Affected babies are usually born unconscious, blind, deaf, and unable to feel pain. Due to the severity of this condition, almost all babies with anencephaly die before birth or only survive a few hours or days after birth. [1,2]

Most occurrences of anencephaly are sporadic and happen in people with no family history of the condition or other NDTs. However, similar to spina bifida, another NTD, low intake of folic acid before and in early pregnancy has been shown to increase the risk of having a child affected by anencephaly. [3] Further supporting evidence for the link between folic acid and NTDs is the significant decline in pregnancies affected by NTDs since the United States began fortifying grains with folic acid. [4]

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

Anencephaly occurs in 2.1 out of every 10,000 live births in the United States. This results in about 860 babies diagnosed with anencephaly nationally each year.[5]

In Alaska, during 2007-2017, the prevalence of anencephaly was 0.7 per 10,000 live births.
Reports Defects Births Prevalence (95% CI)
Total 23 9.0 123630 0.7 (0.4, 1.4)
Notes: 95% CI = 95% Confidence Interval

Trend

Prevalence per 10,000 births of anencephaly during 2007-2017 by five-year moving averages, with 95% confidence interval band and Poisson estimated fitted line.
Reports Defects Births Prevalence (95% CI) Predicted Prevalence†
2007-2011 3.0 1.2 11404.7 1.0 (0.1, 3.8) 0.7
2008-2012 1.7 0.7 11354.0 0.6 (0.0, 3.2) 0.7
2009-2013 1.3 0.5 11349.0 0.5 (0.0, 3.2) 0.6
2010-2014 1.0 0.4 11334.7 0.3 (0.0, 3.2) 0.6
2011-2015 1.3 0.5 11377.3 0.4 (0.0, 3.2) 0.6
2012-2016 1.7 0.7 11295.0 0.6 (0.0, 3.3) 0.5
2013-2017 2.0 0.8 10979.7 0.7 (0.0, 3.4) 0.5
Notes: Each row is based on five-year moving averages; Prevalence reported per 10,000 live births; 95% CI=95% Confidence Interval

† Estimated rate based on Poisson model
The p-value test for trend detected no significant increase or decrease in the number of live births with anencephaly during 2007-2017. See p-value estimate
Estimate Std. Error t value Pr(>|t|)
-0.05868 0.07219 -0.81276 0.45330

Regional Distribution

Distribution of anencephaly 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 12 4.7 50455 0.9 (0.3, 2)
Gulf Coast - - 7725 -
Interior - - 22487 -
Mat-Su - - 14937 -
Northern - - 8520 -
Southeast - - 7625 -
Southwest - - 11881 -
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 anencephaly. 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 12 4.7 59998 0.8 (0.3, 1.7)
  Male 11 4.3 63632 0.7 (0.3, 1.6)
Birth weight (grams)
  <2500 7 2.8 7231 3.8 (0.9, 10.0)
  2500+ 15 5.9 116215 0.5 (0.2, 1.0)
Maternal age
  12-19 - - 9156 -
  20-24 6 2.4 32872 0.7 (0.2, 2.2)
  25-29 7 2.8 37743 0.7 (0.2, 1.9)
  30-34 - - 28152 -
  35-39 - - 12614 -
  40+ - - 3064 -
Maternal race
  Alaska Native/American Indian 9 3.5 31560 1.1 (0.3, 2.8)
  Asian/Pacific Islander - - 11810 -
  Black - - 5081 -
  White - - 73595 -
Maternal education (years)
  <12 - - 11929 -
  12 - - 43162 0.6 (0.1, 1.7)
  12+ - - 65105 0.7 (0.2, 1.6)
Marital status
  Married 9 3.5 78396 0.4 (0.1, 1.1)
  Unmarried 14 5.5 44625 1.2 (0.5, 2.6)
Maternal smoking use
  Reported smoking - - 16817 -
  Reported not smoking - - 104707 0.7 (0.3, 1.4)
Medicaid (mother or child)
  Medicaid 16 6.3 62666 1.0 (0.4, 2.1)
  non-Medicaid 7 2.7 60839 0.4 (0.1, 1.2)
Father on birth certificate
  None 6 2.4 6328 3.7 (1.0, 11.4)
  Present 17 6.7 117302 0.6 (0.2, 1.1)
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] Centers for Disease Control and Prevention. Facts about Anencephaly, https:// www.cdc.gov/ncbddd/birthdefects/anencephaly.html; 2015 [accessed 03.06.2017]

[2] NIH National Center for Advancing Translational Sciences. Genetic and Rare Diseases Information Center: Anencephaly, https:// rarediseases.info.nih.gov/diseases/5808/anencephaly#ref_9565; 2015 [accessed 03.06.2017]

[3] Oakley GP Jr. Folic acid-preventable spina bifida and anencephaly. JAMA 1993; 269(10): 1292-3.

[4] Centers for Disease Control and Prevention. Updated Estimates of Neural Tube Defects Prevented by Mandatory Folic Acid Fortification - United States, 1995-2011. MMWR Morb Mort Wkly Rep. 2015: 64(01); 1-5

[5] Centers for Disease Control and Prevention, Birth Defects Data and Statistics, https://www.cdc.gov/ncbddd/birthdefects/data.html; 2016 [accessed 02.23.2017]

Authorship

Maternal and Child Health (MCH) senior epidemiologist Dr. Jared Parrish, PhD conceived of the presented analysis. Alaska Birth Defects Registry program manager and epidemiologist Chris Barnett, MS MPH and Dr. Jared Parrish, PhD developed the theory and performed the computations. Research analysts Monica Mills and Jordyn Lord managed data collection and storage. All authors discussed the results and contributed to the final report.

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: Anencephaly, Alaska, 2007-2017. Updated June 22, 2021. Available at: http://rpubs.com/AK_ABDR/Anencephaly07_17.

Contact

Alaska Birth Defects Registry (ABDR)
3601 C Street, Suite 358
Anchorage, AK 99503
(907) 269-3400 phone
(907) 754-3529 fax

Updated: June 22, 2021
Code source: R:\ABDR\Analysis_New\ABDR_CASECONF\cond_reports\Published_reports\Anencephaly07_17.Rmd