I hada similar experience. I was over 20 weeks when we found out but I don't know by how much. The difference is that I had been to the doctor numerous time saying I felt pregnant, despite having had a period every month (as the OP said, they were lighter but still realistic as periods), having taken a number of negative pregnancy tests and being on the pill. I felt very sick but lost a lot of weight and felt incredibly tired. Doctors did numerous blood tests and even suggested leukemia but always refused to test for pregnancy, even though I begged them to
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At one point I was leaking milk from my boobs, quite a lot, and they still said I couldn't be pregnant, due to the pill, negative tests and regular periods.
Within a couple of weeks of that I sprouted a bump what felt like very suddenly - one day flat stomach, the next round and hard, and started feeling movements. Fortunately this time when I went to the doctor there was a new one, a locum I hadn't seen before. He went straight to listen for a heartbeat and found one immediately. He said that in his opinion I would be too late for the legal termination date. I was terrified and panicked. I pointed out that on my notes it would show I had visited the doctor at least eight times asking them to confirm a pregnancy and that despite at least five blood tests, none had done so. I then threatened to leave his office and go straight to the nearest lawyer to start some kind of legal action, and also to the press.
Given that my notes were very clear and showed I had been insistent and been ignored every time, he agreed that previous doctors had been extremely negligent and managed to get me an abortion booked in for within two days. I don't know how far along I was and while I'm not proud of happy about having had to abort, I know it wouldn't have been the right thing to have the baby.