Our expert says:
The blood pregnancy test is based on measuring the hormone, HCG. This is produced by the pregnancy until about 16 - 20 weeks. Thereafter the placenta takes over the support of the pregnancy. Late in pregnancy, therefore, the pregnancy test may be negative but, by this stage, the pregnancy is usually obvious on examination. Your friend was obviously quite far along when she ahd the blood test. Before 16 weeks, the blood test is extremely accurate.
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