The federal deficit hit $174 billion for the first four months of fiscal year 2018, the Congressional Budget Office projected Wednesday, up $16 billion from the same period a year ago.

The deficit grew because spending grew 5 percent, while revenues increased 4 percent.

The fact that the deficit rose in the first four months doesn't necessarily mean that it will be higher on the year. The budget office's most recent projections suggested that the shortfall would actually decline in 2018, after hitting $666 billion in 2017.

The deficit could come in larger, though. In the intervening time since that projection, Republicans enacted a tax law that is estimated to add over $130 billion to the deficit in 2018. And Senate leaders from both parties announced a deal Wednesday to increase spending on defense and domestic government programs.

The deficit is the difference between government spending and government receipts in a given year. The federal debt represents accumulated deficits.

Fiscal year 2018 was supposed to be the last year in which the deficit would fall, according to the CBO. In all the subsequent years, it was projected to rise.