Forty years after the Supreme Court opened the gates to legalized abortion, a majority of Americans seem to support Roe v. Wade, though many supporters still struggle with the finer moral details.

A telephone poll of 1,000 adults conducted by NBC News and the Wall Street Journal found that 70 percent of respondents did not want to see the Supreme Court completely overturn the Roe v. Wade decision. However, the poll also found that just 54 percent of respondents thought that abortion should “be legal most of the time” or “always be legal.”

“Abortion is deeply personal, often complex and not something that can be put squarely in a ‘pro’ or ‘anti’ box,” Katherine Humphrey, the president of Planned Parenthood of West and Northern Michigan, wrote in a column for MLive. Indeed, the number of Americans who support access to safe and legal abortion is consistently higher than those who identify as pro-choice. And many Americans self-identify as both pro-choice and pro-life or neither.

In the four decades since the Supreme Court’s decision, much research has been done on unplanned pregnancies, the deaths of mothers in childbirth and fetal development, yet scientists still have yet to come to a consensus on the thorny bioethical issues associated with abortion.

One of the stickier issues that arises is whether or not fetuses can feel pain and, if so, at what point in development that arises. In 2010, the state of Nebraska banned abortions after 20 weeks, citing some studies showing that fetuses can feel pain at that time. Other states, including Kansas, Indiana and Oklahoma, passed similar bills.

But researchers that have reviewed the literature say that fetuses younger than 24 weeks old simply do not have the brain wiring setup required to feel pain. Shortly after the Nebraska bill was introduced, the UK-based Royal College of Obstetricians and Gynecologists put out a working report that said doctors do not need to administer painkillers to fetuses before 24 weeks gestation, because connections have not yet formed between the nerves and the cortex.

Advocates of fetal pain bills have pointed to evidence that fetuses exhibit responses to painful stimuli.

“There are indeed reflex responses, but in our view, because the nerves are not wired up to the cortex, they are reflex actions without experience of pain,” Allan Templeton, who led the group that produced the RCOG report, told New Scientist in 2010.

University of Tennessee researcher Kanwaljeet Anand disagrees, saying that younger fetuses still feel pain, but in a primitive part of the brain called the subcortex.

"It's excruciating," Anand told Discover magazine. "Not only is sensitivity to pain higher in the fetus, it doesn't know when the pain is going to end."

Templeton and other scientists don’t think the subcortex can deliver the same pain experience that the cortex can. Plus, Templeton points out, Anand’s theories are mostly predicated on observations of premature babies.

"He has written opinions about how that might apply also to fetuses, but it's not evidence, it's opinion,” Templeton told New Scientist.

One of the larger questions of when a developing fetus becomes conscious is, like the fetal pain issue, bound up in the development of the brain. The thalamo-cortical complex, a brain area heavily implicated in consciousness, puts its network in place between 24 and 28 weeks gestation. In the womb, the fetus is also pretty much constantly asleep, kept sedated by low oxygen pressure and naturally sleep-producing substances generated by the placenta and the fetus.

The judges that decided Roe v. Wade, however, rested their case on privacy issues, not so much on questions of science.

“We need not resolve the difficult question of when life begins,” Justice Harry Blackmun wrote in his 1973 majority opinion. “When those trained in the respective disciplines of medicine, philosophy and theology are unable to arrive at any consensus, the judiciary, at this point in the development of man's knowledge, is not in a position to speculate as to the answer."