I read somewhere that since the 1970's there have been 150,000 abortions. These people would be in the workforce now, providing funds for the social security system, paying taxes, etc. However, since they are not here, we are lacking those funds. Infer what you want from that...
Cause and effect relationships are difficult to establish with regards to an entire society, especially over decades, so I have to agree with others that you should use caution in drawing any sweeping conclusions. You might also read the chapter in the book Freakonomics which suggests, quite controversially, that crime rates in the 1990s dropped because of the Roe v. Wade decision in the early 1970s. Agree or disagree, he has some interesting research you might be able to use.
Looking at the question you are tempted to come up with a one sided answer: legalized abortion means more abortions which means a lower birth rate. Obviously this is too simple a look at the question. It ignores all the other factors that play into a nation's birth rate. I question any statistic that claims to relate a change in the live birth rate directly to the legalization (or prohibition) of abortion. There are far too many other factors for any relationship to be more than a correlation, not a cause and effect situation. One of the factors you have to take into consideration is that Roe v Wade went into effect during the same era that the contraceptive pill became available and accepted. How can we know which affected the birth rate more or less? Clearly there is a correlation between the legalization of abortion and the live birth rate, but any statistic that claims a direct cause/effect relationship and doesn't take into account the other variables is biased.
If you want a quite thorough discussion of birth rates and their correlation to the legalization of abortion in major cities, please consider reviewing Chapter 4 of Freakonomics. According to Steven Levitt's research presented in the chapter, during the first year after Roe v. Wade was finalized, there was 1 abortion for every 4 live births (750,000 reported abortions). By 1980, 1.6 million abortions were reported, making the ratio 1 abortion for every 2.25 live births. Since 1980, the abortion-live birth ratio has remained basically the same (this information appears on page 138 of the paperback edition of the book).
Abortion and crime are related, even though it is an unpopular idea. Malcolm Gladwell discusses this in The Tipping Point. Many people don't agree, but it's something to look into to support your case. There is a larger number of unwanted births among individuals living in poverty. However abortions are costly, and these individuals probably can't afford them anyway.
I have to agree with #2 here. You need to be very careful at pointing towards statistics like that as a way of talking about the impact of abortion. There are many other factors that we can point towards to explain the decrease in birth rate. You might benefit from investigating other statistics such as the rate of abortions if you can get hold of such figures.
The birth rate in the US was 14.9 live births per 1000 population in 1973, the year that Roe was handed down. It was 14.0 per 1000 in 2009.
You need to watch out, though, about attributing that change to abortion. There are many factors that need to be considered. On the one hand, America has gotten richer and women are working more. Both of those tend to depress birth rates. On the other hand, there are more immigrants now than there were in 1973 and immigrants tend to have higher birth rates. These two facts pull in different directions -- one would make abortion less responsible for the change and one would make it more so.
So the birth rates are a piece of data that can't be disputed. But the interpretation is clearly disputable.