With assisted reproductive techniques, we do know when the patient became pregnant. On many occasions, the date of the last menstrual period will not coincide with the date of the actual presentation, since, due to the action of hormones, this calculation may change, which we call modified or corrected LMP.
After an in vitro fertilization (IVF) cycle with fresh transfer, fertilization occurs on the same day as the follicular puncture, i.e. the oocyte aspiration. Embryologists inseminate the oocytes with the semen a few hours after the oocytes are retrieved. Therefore, to calculate the date of the last menstrual period, you must subtract 14 days from the day of oocyte retrieval, regardless of the day of transfer.
After an embryo thawing cycle, the calculation changes. If the embryo was 3 days old when vitrified and is devitrified on the same day of transfer, we subtract 17 days from the day of transfer and thus calculate the FUR. If the embryo was 5 or 6 days old when frozen and thawed on the same day of the transfer, then we subtract 19 days from the day of the transfer and thus we can calculate the date of the last menstrual period.
The endometrial preparation treatment does not interfere in these calculations, it is the same if the cycle was performed with administered hormones, which we call a “substituted” cycle, that is, a natural cycle that takes advantage of spontaneous ovulation.
The expected date of delivery may be calculated using different methods, depending on the type:
- IVF/ ICSI with own eggs: uses the date of egg retrieval and then adds 266 days (or 38 weeks).
- IVF/ ICSI with fresh donor eggs: uses the date of egg retrieval and then adds 266 days (or 38 weeks).
- 5-day frozen embryo transfer: use the extraction date and then add 266 days (or 38 weeks). Then subtract 5 days (for blastocysts). If the age of the blastocysts is greater than 5 days, subtract their exact age. Another simple alternative is to use the transfer date and add 261 days.
- Frozen embryo transfer:we add 266 days (or 38 weeks) to the date of the embryo transfer. Then, we subtract 5 days (if transferred in blastocysts) or 3 days (for 3-day embryo transfers). This would then add 261 or 263 days to the date of transfer.
