During the study period (1988-2006), a total of 1,225 cases (1,115 live births, 72 stillbirths and 38 terminations of pregnancy) with CAs were ascertained. Table 1 presents the total number of CAs, the total, and live birth prevalence. The overall total prevalence of CAs was 25.72 (95% CI 24.3 to 27.15) per 1,000 births over the study period. There was a significant increase in total prevalence from 17.76 (95% CI 13.45 to 22.07) per 1,000 births in 1988 to 29.40 (95% CI 22.41 to 36.39) in 2006 (c2 = 5.03; p = 0.025). There were 47,229 live births and 1,115 live births with CAs, giving an overall live birth prevalence of 23.61 (95% CI 22.24 to 24.98) per 1,000 live births. There was a significant increase in live birth prevalence from 16.45 (95% CI 12.29 to 20.62) per 1,000 live births in 1988 to 26.18 (95% CI 19.53 to 32.84) in 2006 (c2 = 4.27; p = 0.039).

Table 2 summarizes the main types of CA; number, proportion (% of all cases registered) and prevalence (per 1,000 registered births). Higher rates were notified for CHD (4.3 per 1,000), all nervous system CAs (3.3 per 1,000), limb defects (2.5 per 1,000). Neural tube defects showed a total prevalence of 2.0 per 1,000 per registered births, and about 20% of all cases of this defect resulted in termination of pregnancy. Most of the CHD cases (80%) were ascertained in the nenonatal period of live born babies or in stillbirths (based on active screening). About 20% of the cases were diagnosed in the post neonatal period up to the end of the first year of life of the babies (some cases were identified via case records from the Department of Pediatrics, University Hospital, Pleven, Bulgaria).

The total prevalence of most CAs remained stable over the study period. However, there was a trend towards a significant increase in the total prevalence of specific anomalies: digestive system anomalies (c2 = 4.48; p = 0.034), tumors (mostly teratomas) (c2 = 4.98; p = 0.026), musculoskeletal and connective tissue anomalies (mainly gastroschisis) (c2 = 5.52; p = 0.019), non syndromal dysmorphologic conditions (c2 = 6.77; p = 0.009).

Table 3 summarizes the main causes of registered CAs, number of cases, proportion (% of all cases registered) and prevalence (per 1,000 registered births). The most common CAs were those due to genetic causes (chromosomal, single gene, multifactorial), with a total prevalence of 15.8 per 1,000 registered births over the study period.