Postpartum depression can end in baby death if untreated.

Byline: Michael Precker and Michael Granberry

DALLAS _ The birth of a baby is supposed to be one of life's most joyous events, a culmination of hope and love and wonder.

So how can it be a trigger for depression _ or even unthinkable tragedy?

"If you haven't lived through it or watched someone who has, it is so hard to fathom," said Ann Dunnewold, a Dallas psychologist. "Becoming a mother is supposed to be the most wonderful time in your life, but it's really one of the most difficult transitions."

The killing of a 10-month-old Plano, Texas, girl on Monday, apparently by a mother suffering postpartum depression, was the third high-profile tragedy of its kind in Texas in the last several years. Last year a mother in east Texas killed her three children by bashing them with stones. In 2001 Andrea Yates of Houston drowned her five children in a bathtub.

Such extremes are rare. Postpartum psychosis affects one-tenth of one percent of new mothers, according to the National Mental Health Association. And they are not new _ although media attention may make it seem more common, such killings go back to ancient times.

But although recent horrific incidents have heightened awareness of…