The trial of Dr. Kenneth Edelin, a Boston City Hospital obstetrician charged with manslaughter in the death of a human fetus, will probably open this afternoon in Boston with the beginning of jury selection.

Both defense and prosecution predict a trial of perhaps a month's duration that could depend on a determination of the age at which a fetus becomes a human being and on considerations of medical ethics and complicated surgical procedures.

Newman A. Flanagan, assistant district attorney in Suffolk County, and William P. Homans Jr. '41, Edelin's attorney, have promised to bring medical authorities from across the nation to testify before Judge James B. McGuire's superior criminal court.

Edelin's indictment springs from an abortion he performed in October 1973. The operation was a hysterotomy, similar surgically to a caesarian section.

Flanagan has indicated that he will argue that the operation could have been executed as a delivery, and not as an abortion.

At issue is the age of the fetus. Flanagan alleges that the fetus was between 24 and 28 weeks old and could have lived.

Still pending before McGuire are several pre-trial motions by the defense, including one to dismiss the indictment and another to re-summons a pre-jury panel.

Homans argued yesterday that the grand jury that returned the indictment against Edelin last spring was illegitimate because it did not represent a cross-section of the potential jurors in Suffolk Country.

Two-to-One Ratio

The computer program that chose the jury panel from which the grand jury in turn selected was designed to create a male to female ratio of two to one.

Homans said that because the computer program was weighted, the resulting jury was not representative of the legal jury pool.

That grand jury was composed of 14 men and four women.

Another defense motion asks that the jury panel of 150 for the Edelin trial be stricken, and a new panel summonsed.

Homans said yesterday that the present jury panel is the "result of the same process."

More than two-thirds of that panel is male.