宝宝比实际孕周大是什么原因

你好，双顶径估算：13周2.52士0.2514周2.83士0.5715周3.23士0.5116周3.62士0.5817周3.97士0.4418周4.25士0.5319周4.52士0.5320周4.88士0.5821周5.22士0.4222周5.45士0.5724周6.05士0.5025周6.39士0.7026周6.68士0.6127周6.98士0.5728周7.24士0.6529周7.50士0.6530周7.83士0.6231周8.06士0.6032周8.17士0.6533周8.50士0.4734周8.61士0.6335周8.70士0.5536周8.81士0.5737周9.00士0.6338周9.08士0.5939周9.21士0.5940周9.28士0.50