A couple in US welcomed a rare set of twins just over a year after they had their first twins. Britney Alba gave birth to monoamniotic-monochorionic twins, also known as 'MoMo' twins. Such twins share the same placenta and amniotic sac in the womb. The rare pregnancy has only 1 in 35,000 to 1 in 60,000 chance in the US.