Under the Canon Code Murder is covered by canon 1397, which is the one the precedes the Canon rule on Abortion Rule 1398:
Can. 1397 A person who commits a homicide or who kidnaps, detains, mutilates, or gravely wounds a person by force or fraud is to be punished with the privations and prohibitions mentioned in ⇒ can. 1336 according to the gravity of the delict. Homicide against the persons mentioned in ⇒ can. 1370, however, is to be punished by the penalties established there.
Can. 1398 A person who procures a completed abortion incurs a latae sententiae excommunication.
Notice, that while Rule 1398 (Abortion) clearly states that abortion is a latae sententiae excommunication, Canon 1397 (Murder) refers you to Canon 1336:
Can. 1336 §1. In addition to other penalties which the law may have established, the following are expiatory penalties which can affect an offender either perpetually, for a prescribed time, or for an indeterminate time:
1/ a prohibition or an order concerning residence in a certain place or territory;
2/ privation of a power, office, function, right, privilege, faculty, favor, title, or insignia, even merely honorary;
3/ a prohibition against exercising those things listed under n. 2, or a prohibition against exercising them in a certain place or outside a certain place; these prohibitions are never under pain of nullity;
4/ a penal transfer to another office;
5/ dismissal from the clerical state.
§2. Only those expiatory penalties listed in §1, n. 3 can be latae sententiae.Thus Murder can be latae sententiae. The restriction to such a sentence contained in § 2 relates to acts generally left up a hearing, as would be in the case of Abortion (i.e. reserved for Priests and other "religious" as that term is used in the Canon Law i.e. NOT parishioners).
Side note on the concept of "Nullity". The church has a long history of NOT ruling something was Null and Void if the person who did the act was under some-sort of ban, and the person affected by the act did not know of the ban. i.e. if a priest was excommunicated but did a marriage ceremony which as an excommunicated priest he could no longer do, that marriage is still valid. It has nothing to do with this act (the nine year getting an abortion) nor the hypothetical act of a mother killing her baby.
Complete Canon Law:
http://www.vatican.va/archive/ENG1104/_INDEX.HTM