You are called to the operating room. The general surgeons have operated on a woman to rule out appendicitis and the signs of an abdominal pregnancy with an 18 week fetus and placenta attached to the omentum. The best course of action in the case is:
Abdominal pregnancy

Management includes urgent laparotomy irrespective of period of gestation.
The ideal surgery is to remove the entire sac, fetus, placenta and membrane. This can be done if placenta is attached to a removable organ like uterus or broad ligament.
If placenta is attached to some vital organs, it is better to take out the fetus and leave behind the placenta and the sac after tying and cutting the cord with its placental attachment.
Absorption of placenta occurs by aseptic autolysis.
If placenta is left, its involution is monitored by serum hCG and USG.

Dangers related to leaving placenta attached:

Infection and abscess
Adhesions
Intestinal obstruction
Wound dehiscence
Removal of both fetus and placenta
Laparoscopic ligation of umbilical cord
Removal of the fetus only
Closely follow until viability and then deliver by lapa- rotomy