In order to answer this question, you are going to need to think about a lot of factors -- it is not as easy as just looking at birth rate statistics.
For example, according to this page (I can't make it link)
The fertility rate in the United States is higher now than it was in 1973. 1973 was the year that the Roe v. Wade decision was handed down, making abortion legal in all states. This rate is gotten by dividing the number of births by the number of women in the US each year. You can also see from the washingtonpost.com link that the birth rates now are higher than they were in 1973.
However, there are problems with this. You can't just say that this proves abortion hasn't affected birth rates. You might need to find out how many women of childbearing age lived in the US in each of these years. What if you find that there are way more women of childbearing age now and yet there are only a few more births per woman? Then you might think abortion had something to do with it.
The links I've provided give you the basic statistics -- the birth rate is not down since Roe. However, you will have to think about why this might be -- you can't automatically say abortion had no impact.