The Wikipedia article explains the decomposition process and at which stage a postmortem fetal extrusion would occur (I added the underlining):
Typically, as a dead body decomposes, body tissues become depleted of oxygen and the body begins to putrefy; anaerobic bacteria in the gastrointestinal tract proliferate and as a result of increased metabolic activity, release gases such as carbon dioxide, methane, and hydrogen sulfide. Bacterial exoenzymes weaken tissues, and increasing pressure forces the diffusion of gases into other tissues where they enter the circulatory system and spread to other parts of the body, causing both torso and limbs to become bloated. These decompositional processes weaken the structural integrity of organs by separating necrotizing tissue layers. Bloating usually begins from two to five days after death, depending on external temperature, humidity, and other environmental conditions. As the volume of gas increases, the pressure begins to force various body fluids to exude from all natural orifices.It is at this point during the decomposition of a pregnant body that amniotic membranes become stretched and separated, and intraabdominal gas pressure may force the prolapse of the uterus, which would result in the expulsion of the fetus through the vaginal canal. The bodies of women who have experienced vaginal childbirth at least once in their lives are more likely to spontaneously expel the fetus during decomposition than those who died during their first pregnancy, due to the more elastic nature of the cervix.What would have prevented Laci's body from decomposing exactly as described? The State argued a "soft kill" as the explanation for lack of forensic evidence at the crime scenes. Dr. Peterson said that the head and limbs were missing because of disarticulation through the process of decomposition, not dismemberment. Dr. Peterson also declared that there was no injury to the uterus, except what occurred through the natural decomposition process. Just exactly what prevented Laci from reaching this point . . . during her decomposition?
The December 1, 2005 issue of Forensic Science, Medicine, and Pathology, includes an article about a postmortem fetal extrusion, "Postmortem fetal extrusion in a case of maternal heroin intoxication."
A 34-year-old heroin addict in the eighth month of pregnancy was found dead in her apartment. The head of a fetus was partly protruding from underneath the woman’s slip. At the time of autopsy, the body was in a state of advanced putrefaction with greenish discoloration of almost the complete body surface showing pronounced marbling and, in addition, now not only the head but also the upper part of the chest of a dead fetus were extruding from the birth canal with head presentation. Autopsy showed no signs of external violence prior to death and, in particular, no indication of preceding manipulations in the region of the obstetrical canal and the uterus could be detected. The uterine cavity showed pronounced putrefactive alterations with the amniotic membranes being partially raised and bloated in a balloon-like fashion. Toxicological analyses revealed acute heroin intoxication of mother and child. The immature neonate showed no signs of live birth. Pathogenetically, the finding of not only the head but also the upper part of the chest of the fetus extruding from the birth canal at the time of autopsy (contrary to the observation made at the death scene that only part of the head of the fetus was protruding) is consistent with post-mortem fetal extrusion caused by putrefactive gas pressure against the pregnant uterus as reported in the earlier German forensic pathological literature. (Abstract)
At autopsy, medical examiners found that both the head and shoulders of the fetus had emerged, and concluded that it was a case of postmortem fetal extrusion in progress. (Wikipedia)Only part of the head was seen extruding at the crime scene, but by the time the autopsy was performed, both the head and the shoulders had emerged -- which shows that postmortem fetal extrusion is a process, just like birth, and not a single explosive burst expelling the baby.
The heroin addict had given birth twice before. In this 2008 case, Wikipedia does not reveal whether the homicide victim had previously given birth.
In 2008, the body of a 38 year old woman, seven months’ pregnant, was discovered in an open field four days after she had disappeared from her residence in Panama. A plastic bag had been left over the head, and she had been gagged; the case was ruled a homicide. The body had suffered from the tropical heat and high humidity, and was bloated and highly discolored. At autopsy, the remains of the fetus were discovered in the woman's undergarments. Although the fetus was in a similar state of decomposition, the umbilical cord was intact and still attached to the placenta inside the uterus. This case demonstrates that clothing will not necessarily hinder the extrusion of the fetus due to decomposition. (Wikipedia)This 2008 case definitely shows that Laci's clothing would not have prevented a fetal extrusion.
The introduction to the article "Postmortem fetal extrusion in a case of maternal heroin intoxication." states that "according to the literature, only fetuses with cephalic presentation are expelled" (pg. 273). Cephalic presentation is also known as vertex position, or head down, and is the normal position for a vaginal delivery. Dr. Esther Tow-Der, one of Laci's OB-GYN doctors, testified that on December 23, 2002, Conner was head down.
There aren't many more ways to say that Conner was head down, but the fact that it was repeated so much ensures that it was not a misspeak on Dr. Tow-Der's part.
The State, by necessity, argued that Laci was killed by a soft kill. The State also argued that Laci disarticulated through the normal decomposition processes, she was not dismembered. On December 23, 2002, Conner was observed to be in the cephalic or head down position, necessary for a fetal extrusion to occur. Two subsequent cases of postmortem fetal extrusion plainly show that in the advanced stages of pregnancy, such as Laci was, the normal decomposition processes will produce fetal extrusion long before the female reaches the state of decomposition in which Laci was found. And the 2008 case clearly shows that clothing will not deter the fetal extrusion.
So why didn't postmortem fetal extrusion happen in Laci's case?
This is just one more absence of the expected that screams out for reasonable people to seriously question this conviction.