Under President Donald Trump, the US started a new chapter in its battle against immigration by barring visas for pregnant women seeking to enter the United States for so-called "birth tourism."Explaining the need to protect what it called the integrity of the Green Card, the White House said foreigners were using visas "to secure automatic and permanent American citizenship for their children by giving birth on American soil."The White House also said the birth tourism industry was a glaring immigration loophole that threatens to overburden valuable hospital resources and is rife with criminal activity.The US Constitution guarantees automatic American citizenship to anyone born on US soil.A White House official said that an estimated thousands of children were born in the United States each year to women who arrived in the country using B visas and the number was rising.In In Depth today, we look at what is the meaning of birth tourism, the extent of the problem, the immigration laws in the US and also why a US Green Card is such a coveted possession for people around the world.