Pregnancy calculations for halachic purposes typically begin with the night of mikveh immersion of the cycle on which a woman conceived and use the Hebrew calendar. For example, 16 Shevat would be the beginning of the ninth month for a woman whose last mikveh immersion prior to conception was on 16 Sivan, which should be very close to the medical count.

Since the practices you mention are voluntary acts of piety and not halachic obligations, there is flexibility in how you make your calculations and they need not be exact.

Please write back with any further questions.