Hi Iam 36 Year Old.I Pregnant By Ivf,they Transfer 3

Hi iam 36 year old.I pregnant by ivf,they transfer 3 embryos.At 22/12/15 in jan 05 my blood test is positive bhcg is 306.with light bleeding.my scan on 19jan they told gestational age of less than 5 weeks with single intrauterine gestational sac seen with good decidual reaction.but my bhcg is 512 on 19 jan. Dr told wait for one week repeat the scan and bhcg.