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The periods may become irregular if your thyroid
function has freshly gone down.
In that case you may need to increase the thyroid hormone under the care of a local clinician.
The HSG test is not a cause for delayed periods but you need to recheck pregnancy
with serum beta hCG
test, to rule it out.
Sometimes the periods can be delayed due to stress
. But in that case they resume in about 10days when stress levels are low.
I hope the advise will be helpful for you.