For example, if you have had in vitro fertilization (IVF), you are calculating your pregnancy based on the date of your embryo transfer and not based on your period or cycle.
Your due date is calculated specifically for your pregnancy using one or more methods.
While pregnancy hasn’t really changed in that time, what has changed is our understanding that individuals have different bodies and may have different circumstances.
For example, the rule may not work as well for women with shorter- or longer-than-average cycles.
For this method to work, you must be receiving prenatal care in the first trimester and you must have an early ultrasound.
Some practitioners do not do a standard early ultrasound.
Because the growth of the baby in the first trimester is fairly standard, even despite wide variation in genetics, it is relatively easy to predict a more accurate due date, without the concern of cycle variations or the inaccuracy of remembering when you had your last period.
This method also works no matter how you conceived.
This could lead to an unnecessary induction of labor if it is thought that the baby is in danger because the pregnancy has gone on too long.
Another group that may need a more nuanced look at its due dates would be pregnant people who used certain forms of fertility help.
It’s not imperative for most women but is an option they can discuss with their midwife or doctor.