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During early pregnancy, ultrasound scans can fail to detect baby/fetal heartbeat in the gestational sac. At about 6-8 weeks of gestation, it gets clearer. Wait and go ahead with getting repeat scan in next 2-4 weeks time (preferably transvaginal ultrasound scan) to look for the fetal pole and heartbeat. You can also consider requesting your gynecologist/obstetrician for getting serial HCG levels tested. 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 should be considered to be started to help with preventing miscarriage/threatened abortion. Take adequate rest. Drink plenty of water. Maintain healthy diet. Avoid any heavy work. Avoid having intercourse during first trimester. Avoid lifting heavy objects and climbing stairs or standing for long periods.
Take care.