It sounds crazy, but a surrogate is considered almost 3 weeks pregnant on the day of the embryo transfer! This must be taken into account when calculating gestational age and due dates. So how do we get to that number?
Due dates in pregnancy are traditionally calculated based on the date of the woman’s last menstrual period. For non-IVF pregnancies, the date of ovulation is assumed to be 14 days after the start of the last menstrual period. We know this is not always 100% accurate, but this is the dating convention used in obstetrics. This means that on this (supposed) date of ovulation, the woman is already 14 days (2 weeks pregnant) even though the embryo has yet to implant in the uterus.
For IVF pregnancies:
- The date of egg retrieval is considered day 14 of the pregnancy. It’s treated as the equivalent of ovulation, even if the stimulation phase was shorter or longer than 14 days.
- Add the number the days that the embryo was growing in the laboratory. This is where it gets a little tricky:
- For fresh embryo cycles: Most commonly, the embryo is allowed to grow for 5 days, but the range can be anywhere from 1 to 7 days.
- For frozen embryo cycles: Add the number of days the embryo was growing in the laboratory before it was frozen (typically this number is 5 days, but 3 days is also common). Ignore the time the period when the embryo was frozen. Also add any additional time the embryo was permitted to grow in the laboratory after thawing (commonly 0-1 days).
So, typically, a surrogate is considered 19 days (14+5) pregnant already right after the embryo transfer.
Keep in mind, however, that often legal contracts will measure pregnancy milestones a different way. Legal contracts will often start counting from the date of the embryo transfer. This is done so there is a clear cut starting date for contract purposes. For example, a surrogate may receive her maternity clothes allowance 10 weeks after the embryo transfer.