However, women are not textbooks and sometimes have shorter or longer cycles and/or have ovulation at slightly different times.
Therefore, knowing when fertilization and conception happen can be a bit tricky.
An obvious marker is the first day of the last menstrual period (LMP). Well, another variable is the length of menses but everyone has a first day so to be consistent, that is the marker used.
By the time they are past 40 weeks, they are ready for induction, despite knowledge of the risks and side effects of induction. There are several cases in which medical management of a pregnancy is based on a knowledge of the specific gestational age of the fetus.
These include: interpretation of pre-natal test results, determination of the best date for a planned cesarean section, decisions about inducing a ‘post-date’ pregnancy.
In this paper, I will not address this emotional and psychological impact in much detail, but this data should be read with an awareness of this, because it certainly impacts the couples that we work with in childbirth preparation classes.
For example, as childbirth educators, we frequently see women who have reached the 38 week of pregnancy, where they’re told the baby is ‘no longer premature’ and the moms are ready for the baby to come.
Inaccurate estimates of fetal age could make this information harder to interpret.
With the current formula, about 2/3 of babies are born within ten days of their due dates, with approximately 80% of babies born sometime between week 38 and 42 after LMP.
In the first case-based class of medical school, students are asked to answer a virtual patient’s question about the development of the fetus. It’s a great example of how detailed facts need the larger context.
These students are smart and they know all about beta Hc G and are anxious to showcase their knowledge of the menstrual cycle with fluctuating levels of various hormones (FSH, progesterone, etc.). The usual dating is gestational age, based on the first day of your last menstrual period.
If you are concerned about development, you don’t start counting at week 3 but start at the time of fertilization, two weeks later.