Oh dear, no worry. Firstly, I haven't heard any report to show the level of 芥蘭 has already exceeded the safety leavel. Vegetable has trace amount level of heavy metals because the soil had been contaminated by pollutant. Why don't you worry other vegetable has contamintants if you worry 芥蘭. Does it make sense? By the way, if you are still scared, you don't eat 芥蘭 anymore.
In contrast, mercury normally is accumulated in great fishes. Therefore, FDA doesn't recommend pregnant women to eat great fishes during pregnancy (you can visit FDA website to see what kinds of fish will harm your fetus).
The drawback of mercury: Mercury will harm your fetus's brain growing.