NEW YORK, Jan 30 (Reuters) - JetBlue Airways Corp. on Tuesday forecast a loss in the first quarter on rising costs, after higher fares helped the budget carrier post a profit in the fourth quarter.

The seven-year-old airline, seeking to control costs, has reined in growth by delaying deliveries of some aircraft and selling others. It has also raised fares at the expense of filling seats as it seeks to improve earnings.

These efforts helped JetBlue return to profit in the fourth quarter. But the results were slightly below Wall Street expectations, and JetBlue's shares fell nearly 3 percent.

The No. 8 U.S. carrier by passenger traffic reported a net profit of $17 million, or 10 cents per share, compared with a loss of $42 million, or 25 cents per share, a year ago. Analysts expected a profit of 11 cents per share on average, according to Reuters Estimates.

Operating revenue jumped 42 percent to $633 million on a 14.5 percent rise in capacity and higher fares. Yield per passenger mile, a reflection of average ticket prices, rose 25 percent in the quarter.

The higher fares dented demand. Load factor -- a measure of the percentage of seats filled by paying passengers -- fell 1.4 points to 79.7 percent.

Despite the improvement last quarter, JetBlue guided toward a loss in the first quarter, forecasting a pretax margin of negative 4 percent to negative 2 percent. It said unit costs were expected to rise 6 percent to 8 percent.

Analysts expected JetBlue to post a profit of 9 cents per share in the first quarter, on average.

But the New York-based carrier expects a profit for the year, forecasting a pretax margin of 5 percent to 7 percent.

JetBlue shares, which hit their highest level in more than two years earlier in January, fell 38 cents to $14.12 in morning trading on Nasdaq.