Why not tell people you would like to put on some weight, then pretend you put on too much by accident?
GET pregnant secretly? As in, without the guy/potential father knowing? Why would you do that to him?
If you mean not telling people that you're pregnant, wear baggy clothes and say you've put on weight. Be a little evasive and non-specific about it. Or you could do what unmarried women did in bygone times: go out of town "to visit an aunt."