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At 6 weeks of gestation, the gestational sac should measure about 25mm and the CRL should be about 5mm or a little more. If during the scan, fetal heartbeat was present, it will confirm the viability of pregnancy. Go ahead with getting serial HCG levels estimated. Doubling up of the hormone levels in 48-72 hours will show that pregnancy is going on smoothly. If levels are not adequate, then external hormonal supplements would be considered to be started to help you in preventing miscarriage/threatened abortion. Repeat ultrasound scan done after 1-2 weeks will also help. Visit your doctor/gynecologist for further evaluation and assistance. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately. Take adequate rest. Drink plenty of water. Maintain healthy diet.
Take care.