Most babies are born without birth defects, but expectant moms still worry. Here's how unlikely some birth defects are, as well as when they're usually diagnosed, since knowing beforehand that your baby may need a medical jump-start can help you meet that challenge.

CONGENITAL HEART DEFECTS (1 in 125 births).

• Diagnosis Occasionally detected during ultrasound in the second trimester and confirmed with fetal echocardiography (an ultrasound of the fetal heart). Most problems are diagnosed at birth.

• Treatment Except for potentially fatal defects, many need only monitoring, whereas others can be corrected with surgery during infancy.

PYLORIC STENOSIS, obstruction in the stomach's lower opening that prevents food from moving into the small intestine (1 in 250 births).

• Diagnosis Usually discovered by age 2 months, when symptoms--projectile vomiting, failure to gain weight--appear; or at an early physical exam.

• Treatment Surgery on the affected muscles can correct the condition.

CONGENITAL HIP DISLOCATION (1 in 400 births).

• Diagnosis Detected at birth or during infancy at a physical exam.

• Treatment A diapering technique that keeps legs slightly apart to stabilize hip joints is often all that's needed; may require a harness (worn for several weeks or months) or surgery.

SICKLE-CELL ANEMIA, an abnormality in the hemoglobin produced by red blood cells (1 in 400 African-American babies).

• Diagnosis Can be detected in the first trimester with chorionic villus sampling (CVS), in the second trimester with amniocentesis, or after birth with blood tests.

• Treatment Daily penicillin.

CLEFT LIP AND/OR CLEFT PALATE (1 in 730 births).

• Diagnosis Ultrasound in the second trimester may detect these facial deformities, or they're apparent at birth.

• Treatment Surgery: for a cleft lip at 3 months; for a cleft palate at age 1.

CLUBFOOT (1 in 1,000 births).

• Diagnosis Sometimes seen on ultrasound during the second trimester, but usually diagnosed at birth.

• Treatment May be possible to correct through exercise. In severe cases, a baby can wear a cast or may need surgery.

DOWN'S SYNDROME (1 in 900 births, although incidence depends on mother's age).

• Diagnosis CVS in first trimester; alpha-fetoprotein (AFP) test and amnio in the second. Researchers are working on new tests of the mother's blood and urine that may be performed during the first trimester.

• Treatment Varies according to severity. Breastfeeding is often recommended to optimize a baby's overall health and development; surgery may be needed for heart defects or vision or gastrointestinal problems.

NEURAL-TUBE DEFECTS, such as anencephaly (the absence of a part of the brain and skull) or spina bifida (in which the spinal column doesn't form properly) (1 in 1,600 births).

• Diagnosis Detected during AFP screenings in the second trimester, followed by ultrasound and amnio.

• Treatment Anencephaly is untreatable; spina bifida may be helped with surgery.

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