To answer this question we have to clarify some concepts beforehand:
We consider that pregnancy begins on the first day of the last menstruation, although fertilization (union of the ovum with the sperm) occurs 14 days later.
The estimated due date is set at 280 days from the start of the last menstrual period. In this way the total duration of pregnancy would be 40 weeks.
The time of pregnancy would be calculated as follows: the day on which the embryos were generated (union of the ovum with the sperm) is considered the 14th day of the cycle. The number of days that the embryos were developed in the laboratory before freezing for subsequent transfer should be taken into account, usually between 3 and 6 days.
If the embryos were 5 days old before freezing, the day of the transfer would be the 19th day of the cycle (14+5), setting the date of the last period 19 days before the transfer. From this date of last period the weeks of pregnancy would be counted.
On the other hand, if the embryos were 3 days old before freezing, the day of the transfer would be considered the 17th day of the cycle (14+3). With this in mind, we would set the date of last theoretical menstruation 17 days before the transfer. From this date of last period the weeks of pregnancy would be counted.