AMSTERDAM, Feb 13 (Reuters) - Aegon, the Dutch-based insurer that does two thirds of its business in the United States under the Transamerica brand, reported on Thursday a bigger than expected fall in underlying pretax profit for the second half of 2019.

Aegon said second half pretax profit was 963 million euros ($1.05 billion), down from 1.01 billion euros in the same period a year earlier, hurt by low interest rates and outflows from its U.S. retirement and annuity business.

A company -compiled analysts poll had put the 2019 figure at 997 million euros. The company's solvency under Europe's Solvency II regime - a figure closely watched by analysts as a measure of an insurer's ability to pay dividends - stood at 201%, up 4 percentage points from the end of July but below analysts' average estimates of 211%.

Departing CEO Alex Wynaendts said the company had faced a "challenging environment" in the second half. He is stepping down in April to be replaced by Lard Friese, the former boss of larger Dutch rival NN Group. ($1 = 0.9200 euros) (Reporting by Toby Sterling; Editing by Muralikumar Anantharaman)