Dramatic projectile vomiting a few weeks after birth is the first sign of pyloric stenosis—overgrowth of the muscular connection between the stomach and intestines, requiring surgical correction. Although not apparent at birth, this condition is believed to have origins in prenatal development, so it's considered a birth defect.

Pyloric stenosis occurs in 1.9 per 1000 livebirths. It is more common in:

	Males (82% of all cases)
	White and Latino/Hispanic infants
	Babies born to younger mothers
	Firstborns (regardless of mother's age).

95% of cases are diagnosed 3-12 weeks after birth (an average of 40 days). Premature births are diagnosed later; however, the estimated time from conception to diagnosis is similar in all births.

About 3% of those with pyloric stenosis are multiple births. Girls are more likely to be affected if they are twins or triplets. Identical twins were both affected 25%-44% of the time, indicating both genetic and environmental factors contribute to this birth defect.

OTHER BIRTH DEFECTS INFREQUENT

91% of children with pyloric stenosis have no additional birth defects, and 3% have a second minor defect (judged unlikely to be related). 7% have other major defects; of these, 13% are diagnosed with a specific syndrome or chromosome abnormality. 3 of 10 infants with Smith-Lemli-Opitz syndrome (a genetic condition causing defective cholesterol production) had pyloric stenosis.

The frequency of kidney or urinary defects in males was 2.7 times higher in boys with pyloric stenosis compared to the rest of our registry. This may reflect greater medical scrutiny, as 1/3 of the additional defects were first noted within 2 weeks of diagnosing pyloric stenosis.