You would be surprised how it can happen. My sister in law 1) had bleeding throughout, she had always been irregular anyway 2) didn't have any nausea 3) is big and carried baby very much in her hips (her mum thought she had lost weight!) 4) would occasionally have pains, this was the baby kicking her, but then of course they would stop!
Anyway when she complained of "cramps" during an evening shift at work (in a shop) her older women coworkers joked, "well, if we didn't know better we'd say you were in labour, ha ha..." She DROVE HERSELF to hospital at 3 am; when they examined her she had FULLY DILATED, my nephew was born shortly afterwards, 7 lb. 15 oz. My brother was in the main waiting area (he didn't know where in the hospital she had been taken) and when the doctor found him and asked, "is your girlfriend in the delivery room?" he looked around to see who they were talking to, and he was the only one in the room ! She couldn't breastfeed, (actually they've just had their second baby and she couldn't feed him either.) Anyway she is a strong lady, two weeks after the birth she was back at college! And everything worked out great, they got married a couple years later...