Definition of "Gestational age"
Gestational age (GA, aka menstrual age, weeks gestation, WG) is a measure of the age of pregnancy, where the origin is the woman's last normal menstrual period (LMP), or the corresponding age as estimated by other models.
- The popularity of using such a definition of gestational age is that menstrual periods are essentially always noticed, while there is usually a lack of a convenient way to discern when fertilization occurred
- The initiation of pregnancy for the calculation of gestational age can be different from definitions of initiation of pregnancy in context of the abortion debate, or beginning of the human personhood
Methods include:
- Adding 14 days to a known duration since fertilization, as is possible in in vitro fertilization. This is based on the precept, that fertilization (aka conception) occurs 2 weeks (14 days) after last menstrual period (LNMP). Thus, it is 2 weeks earlier than conception
- By early obstetric U/S, comparing the size of an embryo/fetus to that of a reference group of pregnancies of known gestational age. If the gestational age calculated from an early U/S conradicts the one from LMP, it is the one from the early U/S that is used for the rest of the pregnancy
- Working backwards from EDD, where gestational age can be calculated using a pregnancy wheel (aka OB wheel), by aligning either the expected due date (EDD, or estimated date of birth EDB) or LMP, and obtaining the week+days for today. Usual cycle length is 28 days, such that:
- Longer lengths, need to be added to the EDD, GA
- Shorter lengths, subtracted to the EDD, GA
- In IVF, calculating days since oocyte retrieval or co-incubation, and adding 14 days
Patient information
Gestational age. Date of conception. Date of fertilization. Last menstrual period. This all seems very confusing!It all starts with conception. That's when the fetus is actually created. Egg and sperm come together. We call that the date of "conception", or the date of "fertilization" - those 2 things mean the same thing. Gestational age on the other hand, is measured from the last menstrual period. What's that? That's when the woman last had her last bleed. Remember that once you have "conception", you're no longer going to have a period. So it's last bleed -> 14 days -> fertilization/conception -> 14 days -> (won't bleed)
I see, but how does "gestational age" relate to "date of conception" then?
Conception usually occurs within 1 day of ovulation. Usually around day 14. On the other hand, we know that menstruation occurs on day 1, and we use that date to calculate gestational age. Therefore, we can say that the date of conception occurs around 14 days after the last menstrual date.
I see, so that's why we say that the LMP is 14 days before the date of conception. But why does gestational age depend on the cycle length?
Keep in mind that the gestational age is calculated from the last menstrual date. Therefore, if you have a cycle length of, rather than 28 days, have a length of 34 days, conception is now not going to occur on day 14. It's going to occur 6 days later (34-28=6), on day 20 (14+6=20). That means you need to add longer cycle lengths to the due date if your wheel is assuming a 28 day cycle. On the other hand, if length is 22 days, conception is now not going ot occur 6 days earlier (22-28=-6), on day 8 (14-6=8). That means you need to add shorter cycle lengths from the due date if your wheel is assuming a 28 day cycle.
So in short, longer lengths are added to due date. Shorter lengths are subtracted from due date. And that depends on how far the cycle length varies from the standard 28 days?
That's correct 
CGA is shorthand for Corrected gestational age, which is the actual age minus the number of weeks the baby was premature. In other words, their birthday is reset to their expected due date (i.e. if born at full term, at 40 weeks). This corrected age is used for premature infants (<34 weeks) when considering development, until the age of 2-2.5yo, of which most will catch up developmentally by this age.
Patient information
How about corrected gestational age, what is that?So we use this when a bub is born PREMATURE, which means <34 weeks.
So we reset it to 40 weeks?
Yep. But we only do this for premature bubs, so if they're 34 weeks or older, we don't do this for them.
So for example, if bub is born at 28 weeks, and they're just born, what is their real age and corrected age?
So if they're born 28 weeks, they're 40-28=12 weeks early. So when they're just born, we don't say that they're 0 weeks old. We actually say that their corrected age -12 weeks old. So 12 weeks down the track, we say that their corrected age is 0 weeks old.
Oh I see, and that makes sense, because you can't expect someone who has been born early to be developmentally up to how long they've been in the world for. It should be based upon how long it's been since conception?
Exactly . So when we say someone is 8 weeks old, we mean it's been 8 weeks since they've popped out of mom. When we say 8 weeks corrected, it means 8 weeks from the date that they were mean to be born; so they're likely in real life, older than that.
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