Top 10 Conditions by Frequency |
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among specific diagnoses with major health impact, Iran TRoCA, 2015-2019 |
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| Condition | Cases | Yearly Avg | Prevalence | |
|---|---|---|---|---|
| Hypospadias | 154 | 31 | 41.0 | |
| Limb deficiency, total | 62 | 12 | 16.5 | |
| A/microtia, unspec. | 60 | 12 | 16.0 | |
| Down syndrome | 52 | 10 | 13.8 | |
| Hydrocephaly | 49 | 10 | 13.0 | |
| Polydactyly preaxial | 41 | 8 | 10.9 | |
| Neural tube defects, total | 37 | 7 | 9.9 | |
| Cleft lip with or without cleft palate | 34 | 7 | 9.1 | |
| Diaphragmatic hernia | 33 | 7 | 8.8 | |
| Esophageal atresia | 32 | 6 | 8.5 | |
Note: not considered here are microcephaly, undescended testis, and unspecified conditions. However, microcephaly and undescended testis are included in the full table below. The green bar visually compares the prevalence (rows) relative to the most common condition in the table. |
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Iran TRoCA, 2015 to 2019
- total births in the 5-year period: 37,552 (livebirths: 37,009)
- average births per year: 7,510 (livebirths: 7,400)
- terminations of pregnancy legal in country: Yes
- data include terminations of pregnancy: Yes
- source structure: Hospital based
A word from the program about the program
Selected data highlights
Programs share aggregated information on selected major congenital anomalies for joint monitoring and evaluation. Programs may include additional anomalies in their activities. The following tables highlight key findings on the congenital anomalies selected for joint monitoring.
Top Ten by Frequency
Because of their frequency, these conditions impact the largest number of individuals and their families and are the most likely to be encountered in clinical practice. These top ten conditions are selected among those with significant clinical and public health impact. Ffor example, undescended testis is not included in this list.
Notable Seven
These conditions were selected based on prior knowledge of their overall health impact (morbidity and mortality), regardless of frequency. This selection is the same for all programs. For example, neural tube defects and critical congenital heart disease are included as they may account in some programs for nearly half of all infant deaths associated with congenital anomalies.
Modifiable risk factors are well established for several of these conditions. This implication, as the potential for primary prevention, is unpacked in a later section (‘what if scenario’).
Seven highly impactful conditions |
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selected on the basis of high morbidity and mortality, and potential for primary prevention |
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| Yearly cases | Percent |
Prevalence |
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|---|---|---|---|---|---|
Program |
Country |
||||
| Neural tube defects (NTD) | |||||
| Neural tube defects, total | 7 | 1174 | 95 | 9.9 | |
| Spina bifida | 6 | 913 | 97 | 7.7 | |
| Anencephaly | 1 | 154 | 80 | 1.3 | |
| Orofacial | |||||
| Cleft lip with or without cleft palate | 7 | 1079 | 88 | 9.1 | |
| Cleft palate without cleft lip | 4 | 605 | 100 | 5.1 | |
| Heart | |||||
| Transposition of great vessels | 5 | 818 | 100 | 6.9 | |
| Hypoplastic left heart syndrome | 5 | 794 | 92 | 6.7 | |
| Tetralogy of Fallot | 3 | 510 | 94 | 4.3 | |
Estimated from program prevalence extrapolated to total country births. Iran births for 2022 from World Bank. The green bar visually compares the prevalence (rows) relative to the most common condition in the table. |
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The full table
This more expansive set of major congenital anomalies, internal and external, includes most conditions of significant clinical and public health impact. Most programs include these conditions, and some even more. Note that a child with multiple anomalies will be counted in all pertinent rows.
A later section provides a more detailed analysis of trisomy 21 (Down syndrome).
Selected congenital conditions by system |
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number of cases and prevalence (prev) per 10,000, for all births and livebirths |
||||||
| All births | Livebirths | |||||
|---|---|---|---|---|---|---|
| Cases | Prev | 95% CI | Cases | Prev | 95% CI | |
| Neural tube defects (NTD) | ||||||
| Neural tube defects, total | 37 | 9.9 | 6.9 - 13.6 | 35 | 9.5 | 6.3 - 12.6 |
| Spina bifida | 29 | 7.7 | 5.2 - 11.1 | 28 | 7.6 | 4.8 - 10.4 |
| Anencephaly | 5 | 1.3 | 0.4 - 3.1 | 4 | 1.1 | 0.3 - 2.8 |
| Encephalocele | 3 | 0.8 | 0.2 - 2.3 | 3 | 0.8 | 0.2 - 2.4 |
| Other brain | ||||||
| Hydrocephaly | 49 | 13.0 | 9.7 - 17.3 | 46 | 12.4 | 8.8 - 16.0 |
| Microcephaly | 24 | 6.4 | 4.1 - 9.5 | 23 | 6.2 | 3.7 - 8.8 |
| Holoprosencephaly | 2 | 0.5 | 0.1 - 1.9 | 1 | 0.3 | 0.0 - 1.5 |
| Eye and Ear | ||||||
| A/microtia, unspec. | 60 | 16.0 | 12.2 - 20.6 | 59 | 15.9 | 11.9 - 20.0 |
| A/microphthalmia, unspec. | 6 | 1.6 | 0.6 - 3.5 | 6 | 1.6 | 0.3 - 2.9 |
| Microphthalmos | 3 | 0.8 | 0.2 - 2.3 | 3 | 0.8 | 0.2 - 2.4 |
| Anophthalmos | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Anotia | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Microtia | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Orofacial | ||||||
| Cleft lip with or without cleft palate | 34 | 9.1 | 6.3 - 12.7 | 30 | 8.1 | 5.2 - 11.0 |
| Cleft palate without cleft lip | 19 | 5.1 | 3.0 - 7.9 | 19 | 5.1 | 2.8 - 7.4 |
| Choanal atresia bilateral | 10 | 2.7 | 1.3 - 4.9 | 10 | 2.7 | 1.0 - 4.4 |
| Heart | ||||||
| Transposition of great vessels | 26 | 6.9 | 4.5 - 10.1 | 26 | 7.0 | 4.3 - 9.7 |
| Hypoplastic left heart syndrome | 25 | 6.7 | 4.3 - 9.8 | 23 | 6.2 | 3.7 - 8.8 |
| Tetralogy of Fallot | 16 | 4.3 | 2.4 - 6.9 | 15 | 4.1 | 2.0 - 6.1 |
| Coarctation of aorta | 9 | 2.4 | 1.1 - 4.5 | 9 | 2.4 | 0.8 - 4.0 |
| Gastrointestinal | ||||||
| Esophageal atresia | 32 | 8.5 | 5.8 - 12.0 | 31 | 8.4 | 5.4 - 11.3 |
| Anorectal atresia/stenosis | 24 | 6.4 | 4.1 - 9.5 | 22 | 5.9 | 3.5 - 8.4 |
| Small intestinal atresia/stenosis | 16 | 4.3 | 2.4 - 6.9 | 16 | 4.3 | 2.2 - 6.4 |
| Genitourinary | ||||||
| Undescended testis | 320 | 85.2 | 76.1 - 95.1 | 306 | 82.7 | 73.5 - 91.9 |
| Hypospadias | 154 | 41.0 | 34.8 - 48.0 | 149 | 40.3 | 33.8 - 46.7 |
| Renal agenesis | 8 | 2.1 | 0.9 - 4.2 | 7 | 1.9 | 0.5 - 3.3 |
| Cystic kidney | 5 | 1.3 | 0.4 - 3.1 | 2 | 0.5 | 0.1 - 2.0 |
| Bladder exstrophy | 1 | 0.3 | 0.0 - 1.5 | 1 | 0.3 | 0.0 - 1.5 |
| Epispadias | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Indeterminate sex | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Limb | ||||||
| Limb deficiency, total | 62 | 16.5 | 12.7 - 21.2 | 62 | 16.8 | 12.6 - 20.9 |
| Polydactyly preaxial | 41 | 10.9 | 7.8 - 14.8 | 41 | 11.1 | 7.7 - 14.5 |
| Limb deficiency, unspec. | 9 | 2.4 | 1.1 - 4.5 | 9 | 2.4 | 0.8 - 4.0 |
| Limb deficiency, transverse | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Limb deficiency, preaxial | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Limb deficiency, postaxial | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Limb deficiency, axial | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Limb deficiency, intercalary | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Limb deficiency, mixed | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Abdominal | ||||||
| Diaphragmatic hernia | 33 | 8.8 | 6.0 - 12.3 | 32 | 8.6 | 5.7 - 11.6 |
| Omphalocele | 23 | 6.1 | 3.9 - 9.2 | 22 | 5.9 | 3.5 - 8.4 |
| Omphalocele/Gastroschisis, unspec. | 7 | 1.9 | 0.7 - 3.8 | 6 | 1.6 | 0.3 - 2.9 |
| Gastroschisis | 2 | 0.5 | 0.1 - 1.9 | 0 | 0.0 | 0.0 - 1.0 |
| Prune belly sequence | 0 | 0.0 | 0.0 - 1.0 | 0 | 0.0 | 0.0 - 1.0 |
| Chromosomal | ||||||
| Down syndrome | 52 | 13.8 | 10.3 - 18.2 | 43 | 11.6 | 8.1 - 15.1 |
| Trisomy 18 | 2 | 0.5 | 0.1 - 1.9 | 2 | 0.5 | 0.1 - 2.0 |
| Trisomy 13 | 1 | 0.3 | 0.0 - 1.5 | 1 | 0.3 | 0.0 - 1.5 |
| Note a dash (-) indicates data not available or not provided | ||||||
A deeper look: comments from the program leads
The program leads provide their insights on data, operations, and recent achievements. Their interpretation of the data is particularly valuable because of their local experience and knowledge.
Total prevalence of anomalies has more than tripled in the region over the past two decades. Early records show that nervous system and genitourinary tract anomalies were the most frequent defects, while later data indicated that heart and limb defects are the most common ones.
There was an unusual increase occurred for the total prevalence of congenital anomalies in 2002 in the registry region. TRoCA, broadly, recorded a low rate for spina bifida, high rates for anencephaly, hydrocephaly, and cleft palate without cleft lip, and limb reduction defects in the region. We do not know for sure whether any change in these rates is due to a true existence/absence of epidemics in the region, due to the technical failure of our monitoring, or due to etiologic and environmental teratogens. The reasons behind the high/low rates of some groups of anomalies are still unclear.
Down syndrome (trisomy 21)
By far the most common chromosomal anomaly, Down syndrome is known to occur more frequently (has a higher risk of occurrence) in births of women with higher maternal age
This pattern is universally observed, provided there is no significant bias toward missing pregnancies with Down syndrome in older women (e.g., because of unreported pregnancy terminations)
Down syndrome, overall and by maternal age |
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separately for all births and livebirths, prevalence per 10,000 (Poisson exact confidence intervals) |
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| All cases | Livebirths | |||||
|---|---|---|---|---|---|---|
Cases |
Prev | 95% CI | Cases |
Prev | 95% CI | |
| All maternal ages | 52 | 13.8 | 10.3 - 18.2 | 43 | 11.6 | 8.1 - 15.1 |
| < 20 years | 2 | 4.9 | 0.6 - 17.7 | 2 | 4.9 | 0.6 - 17.7 |
| 20 to 24 | 7 | 8.0 | 3.2 - 16.5 | 6 | 6.9 | 1.4 - 12.4 |
| 25 to 29 | 4 | 3.8 | 1.0 - 9.8 | 3 | 2.9 | 0.6 - 8.4 |
| 30 to 34 | 14 | 17.0 | 9.3 - 28.5 | 14 | 17.0 | 8.1 - 25.9 |
| 35 to 39 | 13 | 28.9 | 15.4 - 49.5 | 9 | 20.0 | 7.0 - 33.1 |
| 40 to 44 | 12 | 94.5 | 48.8 - 165.1 | 9 | 70.9 | 24.7 - 117.0 |
| 45+ years | 0 | 0.0 | 0.0 - 217.2 | 0 | 0.0 | 0.0 - 217.2 |
| Age unspec. | 0 | - | - | 0 | - | - |
| Age unspec. = maternal age unknown or unspecified | ||||||
Down syndrome - maternal age pyramid
Because of the relation between prevalence of Down syndrome and maternal age, the maternal age distribution in the population is a major determinant of the number of conceptions with Down syndrome in the population.
For programs that have maternal age specific data, one can compare the maternal-age ‘pyramid’ for all births in the population with that of births with Down syndrome. Typically, the distribution is skewed, with a relative excess of births with Down syndrome among the more advanced maternal age groups.
Down syndrome - birth rates matter
Despite the considerably higher risk (rates) of Down syndrome in mothers over 40 or 45 years, these age groups contribute comparatively fewer affected births than younger age groups. This is because birth rates matter: fewer births in the more advanced age groups mean fewer cases overall from those age groups.
A Pareto chart helps highlight the cumulative contribution of different age groups to the total number of cases. This information can help inform strategies for testing and counseling.
Linking risk factor profile and congenital anomalies
The term triple surveillance refers to a model of public health surveillance that births includes the full causal chain, from a) risk factors, which influence the number of affected pregnancies; to b) the affected pregnancies themselves, that are vulnerable to adverse health outcomes; and to c) the health outcomes in affected individuals. The burden of risk becomes expressed eventually in the burden of disease.
Historically, birth defect surveillance has focused on the second element, the occurrence of congenital anomalies.
However, improving outcomes (morbidity, mortality, disability) requires understanding and tracking risk factors (to improve primary prevention) and health outcomes (to improve care). Triple surveillance advocates integrating the tracking of all three elements in this causal chain. Together, these three domains provide clinicians, public health professionals, and policy makers with information to act.
To highlight such context, the next sections provide elements of country demographics, outcomes (mainly early mortality), and selected risk factors. These data, particularly those on risk factors and outcomes, are sometimes directly measured, and sometimes estimated. Birth defect programs with their partners can supplement these data with local assessments.
Country demographics
A surveillance program operates within its country’s demographic situation and trends. This information adds meaning and context to birth defect surveillance information.
Demographic Indicators, Iran |
|
| on population, births, life expectancy | |
| Key Indicator | 2022 data |
|---|---|
| Total population | 89,524,246 |
| Number of births | 1,185,480 |
| Birth rate (per 1000 pop) | 13.2 |
| Fertility (births per woman) | 1.7 |
| Life expectancy at birth (years) | 74.6 |
| Source: World Bank (accessed Sept 2024) | |
From program to country
Many (though not all) programs cover a proportion of the country in which they operate. In this setting, a common question is what the program can tell use about the impact of congenital anomalies country-wide, under the assumption that the program information is a good estimator for the country itself. This assumption, of course, needs to be carefully examined, and the program staff typically has the local knowledge to help frame such estimates within the strengths and limitations of the data.
The table below uses the prevalence measured within the program to estimate the number of births with selected congenital anomalies for the entire country. These extrapolations have limitations, and in most cases are illustrative. However, at times a program that covers a proportion of the population may be the only practical window into congenital anomalies country-wide.
A window into the country: Iran, 2022 estimates |
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Country-wide estimates for selected conditions (1,185,480 births), extrapolating from program data |
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| All cases | liveborn | % liveborn | |
|---|---|---|---|
| Neural tube defects (NTD) | |||
| Neural tube defects, total | 1168 | 1121 | |
| Spina bifida | 916 | 897 | |
| Anencephaly | 158 | 128 | |
| Encephalocele | 95 | 96 | |
| Other brain | |||
| Hydrocephaly | 1547 | 1473 | |
| Microcephaly | 758 | 737 | |
| Holoprosencephaly | 63 | 32 | |
| Eye and Ear | |||
| A/microtia, unspec. | 1894 | 1890 | |
| A/microphthalmia, unspec. | 189 | 192 | |
| Microphthalmos | 95 | 96 | |
| Orofacial | |||
| Cleft lip with or without cleft palate | 1073 | 961 | |
| Cleft palate without cleft lip | 600 | 609 | |
| Choanal atresia bilateral | 316 | 320 | |
| Heart | |||
| Transposition of great vessels | 821 | 833 | |
| Hypoplastic left heart syndrome | 789 | 737 | |
| Tetralogy of Fallot | 505 | 480 | |
| Coarctation of aorta | 284 | 288 | |
| Gastrointestinal | |||
| Esophageal atresia | 1010 | 993 | |
| Anorectal atresia/stenosis | 758 | 705 | |
| Small intestinal atresia/stenosis | 505 | 513 | |
| Genitourinary | |||
| Undescended testis | 10102 | 9802 | |
| Hypospadias | 4862 | 4773 | |
| Renal agenesis | 253 | 224 | |
| Cystic kidney | 158 | 64 | |
| Bladder exstrophy | 32 | 32 | |
| Limb | |||
| Limb deficiency, total | 1957 | 1986 | |
| Polydactyly preaxial | 1294 | 1313 | |
| Limb deficiency, unspec. | 284 | 288 | |
| Abdominal | |||
| Diaphragmatic hernia | 1042 | 1025 | |
| Omphalocele | 726 | 705 | |
| Omphalocele/Gastroschisis, unspec. | 221 | 192 | |
| Gastroschisis | 63 | 0 | |
| Chromosomal | |||
| Down syndrome | 1642 | 1377 | |
| Trisomy 18 | 63 | 64 | |
| Trisomy 13 | 32 | 32 | |
| Note: includes conditions with at least 5 estimated cases. Assumes that prevalence estimates from program are valid country-wide. | |||
Outcomes - early mortality
The relative impact of congenital anomalies on early mortality (neonatal, infant, and under 5 years) tends to increase as infant mortality due to other causes falls. This pattern has been observed worldwide. These general indicators of early mortality are tracked regularly by public health agencies.
Mortality Indicators, Iran |
||
| Key Indicator | 2022 data | |
|---|---|---|
| Neonatal mortality (per 1000) | 7.6 | |
| Infant mortality (per 1000) | 10.3 | |
| Under 5 mortality (per 1000) | 12.0 | |
Source: World Bank, accessed Sept 2024. |
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Outcomes - mortality with congenital anomalies
A more specific indicator (infant deaths due to congenital anomalies) is more challenging to document accurately. Missed diagnoses, especially of internal anomalies, can lead to massive underestimates, especially in settings where diagnoses rely only or mostly on an external exam. The table below summarizes data and estimates from systematic public sources. However, local assessments from birth defect surveillance program can help improve the quality of this key indicator.
Deaths due to birth defects, Iran |
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| among infants (< 1 year old) and from first to fifth birthday | |||
| Age group | Percent | Deaths |
|
|---|---|---|---|
| Infants | 21.5 | 253.3 | |
| 1 to < 5 yrs | 15.0 | 9.7 | |
| Source: WHO mortality data for 2017 | who.int. |
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Risk factors
Some modifiable exposures are well-established risk factors for congenital anomalies. Reducing these exposures is the basis for effective primary prevention. Here we focus specifically on smoking, diabetes, and folate insufficiency. These factors, both common and modifiable, increase the risk of major contributors to morbidity and mortality, including orofacial clefts, neural tube defects, and serious congenital heart disease. The impact of these risk factors depend on their frequency in the population. The following tables show frequency estimates for the country. In another section, these data are used to estimate the number of cases in the country potentially preventable by eliminating the risk factors.
Smoking
Smoking is associated with increased risk for many adverse pregnancy outcomes and several congenital anomalies, including orofacial clefts and probably congenital heart disease. The increased risk is relatively modest (odds ratios tipically less than 1.5) but the effect size, or number of cases due to smoking, depends on the rate of smoking among women who become pregnant. Here we used country-specific estimates of smoking in women of reproductive age (see table for references). Rates of smoking during pregnancy tend to be lower, but by the time many women know they are pregnant, the at-risk period for congenital anomalies has often already passed.
Smoking in women |
|
| Frequency in two different groups, Iran | |
| Women | Frequency (%) |
|---|---|
| Of reproductive age | 5.9 ( 4.2 - 7.9 ) |
| During pregnancy | NA ( NA - NA ) |
| Source: IHME | www.healthdata.org | and Lange 2018 | |
Diabetes
Maternal pregestational diabetes is associated with increased risk for many serious congenital anomalies, including spina bifida, several critical congenital heart defects, and multiple congenital anomalies, among others. In some cases the relative risk (or odds ratio) can be quite high, above 4 or 5 in some cases. The effect size (number of cases due to diabetes) depends on the frequency of diabetes, which tipically increases with age.
Diabetes in women |
||
| Frequency by age group, Iran | ||
| Age Group | Percent | |
|---|---|---|
| 15 to 19 | 1.0 | |
| 20 to 24 | 1.8 | |
| 25 to 29 | 2.7 | |
| 30 to 34 | 4.1 | |
| 35 to 39 | 6.0 | |
| 40 to 44 | 8.6 | |
| 45 to 49 | 11.6 | |
| Source: IHME | www.healthdata.org | ||
Folate insufficiency
Folate insufficiency is associated with increased risk for neural tube defects and perhaps other congenital anomalies. On a population-basis, well-implemented folic acid fortification is estimated to decrease the prevalence of neural tube defects below 6 per 10,000 or perhaps even lower. Folic acid fortification is most effective when it is mandatory and universal, meaning that it involves foods commonly consumed by large parts of the population (e.g., wheat, maize, rice, depending on culture and geography).
Folic acid fortification |
||
| Status of mandatory fortification in Iran | ||
| Mandatory | Food vehicle | Year started |
|---|---|---|
| No | - | - |
| Source: Country program and FFI | fortificationdata.org | ||
What if - prevention scenarios
What follows is a tricky but important exercise. By combining information about risk factors and prevalence of congenital anomalies, one can attempt to estimate the number of cases attributable to risk factors. This is illustrated in the table below, which uses prevalence information from the program and risk factor information (e.g., diabetes frequency in women) from country data. These estimates also require knowing the relative risk of disease given the exposure. Here we use odds ratios derived from high quality studies and metanalyses. These data then are fed into the Levine estimator of attributable fraction to generate the scenarios that you see in the table. For neural tube defects, we used a different approach. We used 6 per 10,000 as the (conservative) estimate of the birth prevalence achievable by fully implemented folate fortification, and postulated that a prevalence in excess of this value consists of cases attributable to folate insufficiency (as relates to neural tube defect prevention).
Attributable cases for selected risk factors |
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yearly estimates in program and in country, and per million births |
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Prevalence |
attributable cases | Risk factor parameters | |||
|---|---|---|---|---|---|
Program |
Country |
per 1M births |
|||
| Neural tube defects (NTD) | |||||
| Neural tube defects, total | 9.9 | 3 | 462 | 390 | Presumed folate insufficiency, prevalence > 6 |
| Orofacial | |||||
| Cleft lip with or without cleft palate | 9.1 | 0 | 21 | 18 | smoking, freq: 5.9%, OR: 1.34 |
| Cleft palate without cleft lip | 5.1 | 0 | 8 | 7 | smoking, freq: 5.9%, OR: 1.22 |
| Heart | |||||
| Transposition of great vessels | 6.9 | 0 | 49 | 41 | diabetes, freq: 2.7%, OR: 3.34 |
| Hypoplastic left heart syndrome | 6.7 | 0 | 70 | 59 | diabetes, freq: 2.7%, OR: 4.58 |
| Tetralogy of Fallot | 4.3 | 0 | 48 | 41 | diabetes, freq: 2.7%, OR: 4.89 |
Odds ratios from literature, exposure frequencies from country estimates (see methods for details). |
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Arguably, these estimates are simplistic. For one, they assume accurate inputs and lack of interactions between risk factors. For neural tube defects, for example, the assumption is that the reported prevalence is accurate (e.g., that it includes stillbirths and pregnancy terminations) and that the excess prevalence is entirely due to (or preventable by) the folate status of the population.
For smoking and diabetes, these estimates assume, for example, that the reported smoking rates in women reflect the smoking rates in the at-risk period, typically the periconceptional period. Smoking rates in pregnancy are typically lower than rates among women of childbearing age, but many women realize they are pregnant after the at-risk period for many congenital anomalies. One may argue that the true smoking rates in the at-risk period are somewhere between these two estimates .
Finally, the computation of attributable cases in these scenarios considers risk factors ‘one at the the time’ and does not account, for example, for multiple exposures (e.g., diabetes and smoking) or interactions among multiple exposures. And so on.
Nevertheless, even rough estimates provide an important message. Some instances, perhaps many, of major and even lethal congenital anomalies are preventable by reducing the burden of risk in populations and individuals. Moreover, for many risk factors (smoking and diabetes in particular), the benefits of exposure mitigation extend beyond the prevention of congenital anomalies to the prevention of many other health conditions, for the fetus as well as for parents.