Donald Trump's first 100 days as the US President are the worst for the US stock market for the start of a president's 4-year term since 1970s. The S&P 500's 7.9% fall is the second worst first 100-day performance since the 9.9% fall under Richard Nixon's leadership. On average, S&P 500 rises 2.1% in first 100 days for any president.