The Philadelphia jury in the murder trial of abortion doctor Kermit Gosnell ended its fourth full day of deliberations Monday and appeared to have begun looking at the murder charges against the 72-year-old physician.

The Common Pleas Court jury of seven women and five men returned to court once in the morning to hear Judge Jeffrey P. Minehart reinstruct, as requested, about the legal definitions of first- and third-degree murder and involuntary manslaughter.

The jury must decide whether Gosnell is guilty of first- or third-degree murder in the deaths of four babies allegedly born alive during illegal late-term abortions and killed by him or his staff.

The jury could also acquit Gosnell if it finds the prosecution did not prove its case beyond a reasonable doubt.

The jury is also considering whether Gosnell is guilty of third-degree murder or involuntary manslaughter in the 2009 death of abortion patient Karnamaya Mongar. Mongar, 41, of Virginia, allegedly died of an overdose of Demerol administered by Gosnell's untrained staff.

Minehart explained to the jury that first-degree murder involves the malicious, premeditated killing of a human being. Third-degree murder is a killing without premeditation but involving malice and a reckless disregard for the consequences of the killer's acts. Involuntary manslaughter involves a death through recklessness or negligence but without premeditation or malice on the killer's part.

The jurors have deliberated a total of about 30 hours since getting the case last Tuesday afternoon.

Gosnell, who operated the Women's Medical Society clinic at 3801 Lancaster Ave. in West Philadelphia, could face the death penalty if the jury finds him guilty of any first-degree murder count.

Gosnell also faces 24 counts of performing abortions past Pennsylvania's 24-week gestational age limit, 227 courts of performing abortions without the mandated 24-hour waiting period, and racketeering and corrupt organizations.

Codefendant Eileen O'Neill, 56, of Phoenixville, an unlicensed doctor who worked in Gosnell's family practice, is charged with six counts of theft by deception - posing as a licensed doctor - that form the basis of racketeering and conspiracy counts.

Contact Joseph A. Slobodzian at 215-854-2985, jslobodzian@phillynews.com, or @joeslobo on Twitter.