If you’re trying for a baby or are pregnant you should avoid having more than four cans of tuna a week (with each tin having a drained weight of around 140g) or two fresh tuna steaks (around 140g when cooked). This is because tuna can contain more mercury than other fish, which can affect a baby’s developing nervous system. There’s no limit on how much you can eat if you’re breastfeeding. Canned and fresh tuna doesn’t count as an oily fish, so you can eat this as well as two portions of oil-rich fish each week.