I did not know this!
A spokeswoman for the first lady said Mrs. Trump’s decision to wear a black lace veil known as a mantilla followed Vatican protocol that women who have an audience with the pope must wear long sleeves, formal black clothing and a veil to cover their head.
In fact, every woman in the U.S. delegation wore a veil, including Ivanka Trump, the president’s daughter who converted to Judaism before marriage.
The Vatican’s rules of attire are not strictly enforced. Many women, including high-ranking dignitaries, have visited the pontiff with their heads uncovered, including German Chancellor Angela Merkel in 2015 and Aung San Suu Kyi, Myanmar’s top civilian leader, this month.
But many women wear veils out of respect. Mrs. Trump is Catholic, which likely made accompanying President Donald Trump for a meeting with the leader of the world’s more than 1 billion Roman Catholics all the more meaningful to her.
When a Vatican official handed her a rosary, the first lady immediately gave it to the pope to bless. She also spent time in front of a statue of the Madonna at the Vatican’s children’s hospital and laid flowers at its feet. She also prayed in the hospital chapel and spoke with the children in Italian.
When former first ladies Truman, Kennedy, Nixon, Reagan, Clinton, Bush, and Obama visited the pontiff, they likewise respected the Vatican’s request that they wear long sleeves, formal black clothing and a veil.
It has been noted that Mrs. Trump did not wear a hijab or headscarf in Saudi Arabia. The government did not request that she do so.
In Israel, President Trump donned a yarmulke when he visited the Western Wall, where it is customary, and again at Yad Vashem, Israel’s Holocaust memorial, where it is not required. Trump likely did it out of respect.
Sources:
polination.blog 
http://www.washingtonexaminer.com/melania-trump-speaks-italian-with-hospitalized-children-in-rome/article/2624011
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