It's one of the macabre mysteries in the case of Kermit Gosnell: Why did the West Philadelphia abortion doctor keep the severed feet of fetuses preserved in specimen jars?

In testimony Tuesday, Adrienne Moton, a former worker at Gosnell's Women's Medical Society, told a Philadelphia jury that Gosnell once explained that he did so in case patients requested them for future identification or DNA samples.

But an expert on fetal development, testifying Wednesday at Gosnell's abortion-murder trial, said that was news to him.

"Do you think there is any medical reason to save the foot of a baby?" Assistant District Attorney Edward Cameron asked.

"In my practice, we would have no reason to save the foot, and I've never seen that done," replied Daniel H. Conway, a physician and neonatologist at St. Christopher's Hospital for Children.

Questioned by Gosnell attorney Jack McMahon, Conway acknowledged that the creases on the bottom of the foot are one factor sometimes used to determine the age of a stillborn or premature baby.

But Conway maintained he had never heard of physicians or researchers preserving and saving severed fetal feet.

Prosecutors have cited the dozens of jars of severed baby feet as an example of Gosnell's idiosyncratic and illegal practice of providing abortions for cash to poor women pregnant longer than the 24-week cutoff for legal abortions in Pennsylvania.

In her opening statement to the Common Pleas Court jury, Assistant District Attorney Joanne Pescatore mused that the jars of feet were some kind of bizarre "trophy" Gosnell kept.

Gosnell, 72, is charged with seven counts of first-degree murder - newborns whose spines he snipped with scissors after late-term abortions. He faces the death penalty if found guilty.

Also on trial is Eileen O'Neill, 52, of Phoenixville, an unlicensed medical school graduate who worked as a doctor in Gosnell's clinic at 3801 Lancaster Ave. She is not charged with performing abortions.

Gosnell is also charged with third-degree murder in the death of a Virginia woman, Karnamaya Mongar, 41, who was allegedly administered too much anesthesia during a 2009 abortion.

In addition to the mystery of the fetal feet, Conway also estimated the age of "Baby Boy A," the first of seven who prosecutors allege were born alive and killed by Gosnell.

Baby Boy A was dead after a July 12, 2008, abortion involving a 17-year-old Chester student.

Moton, the former clinic worker, testified that she was so upset at the sight of the large baby's bloody, mangled body that she took a photo with her cellphone.

That case - and Moton's photo - have been the focus of the trial's first three days of testimony.

The Chester woman testified Gosnell told her the fetus was 24 weeks old when she went for the abortion.

Conway, however, said his reading of the photo and ultrasounds of the fetus Gosnell took before the abortion made him estimate the fetus was 27 to 31 weeks old at the time of the abortion.

Conway said at that age, the boy had an 85 percent chance of surviving and thriving into adulthood.

Had the child been born alive but premature, Conway told Cameron, "my duty [as a doctor] would have been to provide care for that infant."

Conway said state law requires doctors to try to save any fetus - even as young as 22 weeks - born prematurely if there are signs of a heartbeat, breathing, or movement.

Questioning Conway, McMahon elicited from the doctor that, based on Moton's photo, there was no way to determine whether Baby Boy A was alive on July 12, 2008.

McMahon argued that the evidence suggested just the opposite, because Gosnell used Digoxin to trigger the abortion.

After the federal government banned the rare, so-called live-birth abortion procedure in 2007, late-term abortion providers like Gosnell switched to Digoxin, a heart drug, to kill fetuses in the womb. The remains are then removed.

"The fact is that baby was not alive because the mother was injected with Digoxin to bring about fetal demise," McMahon said.