Because of its caffeine content, it should be monitored - and it should be consulted with a doctor.
There are different recommendations in terms of what is the limit of caffeine per day during pregnancy (it varies between 200 and 300 mg). This includes mate but also coffee, tea, sodas, etc.
A 2005 study showed that drinking mate on a daily basis whilst being pregnant had ‘no harmful effect on intrauterine growth or duration of pregnancy’. However, an Uruguayan study done in 2014 concluded that ‘The consumption of more than 300 mg a day is associated with a higher risk of small newborns for their gestational age.’
It is also encouraged to avoid the first brews (as those are higher in caffeine) or have mate cocido (boiled mate). As mentioned before, different types and brands of yerba mate have different caffeine content.