didn't know she was pregnant until she was 6 months

My mother knew she was pregnant but the tests came back negative but she said she had a strong feeling she was and no one was going to change her mind. She went to take a blood test and it came back negative too I was starting to doubt wether she was just getting really stressed, physically she was getting big but week after week the results were still negative. Yet, she never gave up last week she went in and demanded an ultrasound and it turns out she's 5 1/2 months with a boy.. yes she is almost 40 but she's been trying since she lost her twins 6 years ago and I couldn't be more happy for her😊