Self Sacrifice
Reiko attended a college lecture on self sacrifice a little before her death. It's interesting because it was basically foreshadowing events that later took place. Self sacrifice is an important theme for the series, and it is especially important for Reiko. Self sacrifice was something that Reiko thought was :irrational". When she ended up sacrificing herself for her baby, I never saw it coming, and that made me appreciate Reiko so much more.
The lecturer basically said that it is not uncommon for parents to protect their young. Lots of species, from wolves to insects put themselves in danger for the benefit of the pack. He used ants and bees as an example.
When an enemy is near, many of them sacrifice themselves to protect the nest. They don't have enough brainpower to think, so why do they act so heroically?
He went on to say that it was genes. There was a popular theory that self sacrifice was the result of an instinct to preserve ones species, but it was rebutted by the killing of children. Monkeys, lions, and certain insects kill their young, probably in the pursuit of ideal genes.
The parasites were similar in that nature. If a parasite didn't take control of a human brain, it was considered a failure by its species, and the other parasites felt the need to kill it. In that way, parasites could be considered sacrificial, but were they self-sacrificial? Reiko clearly was, but she was an exception.
I would say other parasites definitely were not. Each parasite did what it had to do to keep on surviving. Most of them thought of no one but themselves. I think the fact that Reiko was so interested in why different species of animals, particularly humans, would be self-sacrificial shows how foreign the idea is to her species. She thought the idea was irrational, and she wanted to learn more. That's most likely why she sat in on that lecture. If it was so irrational, though, then why did Reiko die for her baby?
I think the answer is in the question. She died for her baby. Parasite or not, Reiko gave birth to the human child, so she was a mother. I think Hitoshi Iwaaki viewed mothers as self-sacrificial.
The self-sacrificial mother is fairly controversial archetype. Some people believe it's sexist, saying that fathers can save their kids, but mothers only die for them. They ask, what about the mothers who save their kids and live in the end? I don't think Iwaaki was sexist. Regardless, the archetype was present in Parasyte. There are flashbacks with Shinichi's mother saving her son from a hot pot falling from the stove and suffering severe burns so that Shinichi wouldn't be hurt. Shinichi's mom was a self-sacrificial mother just like Reiko. At least in Parasyte, mothers were.
And so in my opinion, Reiko was definitely not self-sacrificial because of her species like the animals brought up in the lecture she attended. I think it was more of a character trait inspired by the archetype Iwaaki used for mothers in Parasyte.