Due date calculations

[leoeps1982]

Hi all,

This question/issue has been bugging me for a while. In US (and pretty much everywhere else, as I can tell) your due date is set to be 40 weeks after the first day of your last period. Your gestation weeks are also counted from the first day of your last period.

When I talk to my mom, about this dating method, she always gets confused, because when she gave birth to me in the old USSR, they counted gestation weeks from the approximate moment of ovulation. The due date was considered to be 40 weeks from ovulation, so that is 42 weeks in the modern count! She always insisted that I was born exactly on my due date, 40 weeks after ovulation. However, it seems that by modern counts I was born at exactly 42 weeks, 2 weeks overdue! So whenever she asks how far along am I, and I tell her, she always subtracts 2 weeks.

Anyways, I tried to do some research on this, and I found something interesting. There are several studies which indicate that Caucasian women, who give birth for the first time, have on average a gestation period 8 days longer then the EDD. Some even suggest to add 15 days to the EDD for a Caucasian woman giving birth for the first time. This is one such study: "The length of uncomplicated human gestation", Obstet Gynecol. 1990 Jun;75(6):929-32. (Note, this study only considered women with reliable menstrual histories, since obviously variations in cycle lengths, late ovulation, etc will impact this.)

Now, I am fully convinced that I will go 1 or 2 weeks over my EDD.

Any thoughts?

 

[BecksBabyB]

Thinking about the ladies I know who have had babies, it makes a lot of sense. I'm expecting to go 'overdue' having had a relatively normal pregnancy. I'm due 29th May but I think it'll be a June baby x

 

[Anthrogirl]

It's a common misconception that 'overdue' is after the 40 week mark. Technically "term" is 37-42 weeks (it used to be 38, but now docs consider a 37 weeker to be 'full term'). A woman is not actually overdue until post 42 weeks.

 

[special_kala]

Your actual due date is from your 12 week scan which is more accurate.

How did they know the exact point of olulation? It was much more of a guess before scans,

 

[Mamoosca]

I agree with this theory. My last baby came right on time considering my ovulation date, using my scan date I was 11 days over.

At the moment I'm 4 days overdue but going by my ovulation date (and the only possible time we could have conceived), today or tomorrow is my due date x

 

[Powell130]

Your actual due date is from your 12 week scan which is more accurate.

How did they know the exact point of olulation? It was much more of a guess before scans,

My perinatologist (high risk OB) told me that my early scans (5w2d & 6w2d) are even more accurate than a 12 week scan. But of course, not everyone gets a scan that early lol

 

[atayl16]

My 1st was 9 days late

 

[Anthrogirl]

Using ultrasound to date a pregnancy only works if you have an ultrasound during your pregnancy!!

 

[LoraLoo]

I dont like the idea of that theory at all. The placenta can start to fail after 41/42 weeks. If pregnant women were allowed to go even longer over the 42 weeks id be very worried.

 

[SparkleUK]

Stalking this thread! V interesting x

 

[leoeps1982]

Well, of course ultrasound dating is more accurate then any guesses of ovulation or when period started. But this isn't a question of how late in the cycle you ovulated, or how far apart your periods were. Put in other words, this is a question of whether to count EDD as being 40 or 42 weeks from your last menstrual period.

Back when my mom gave birth to me, they didn't know exact ovulation dates, they just started counting weeks from the midpoint of a woman's cycle, not from first day of period, and they still counted 40 weeks. There were no ultrasounds back then (at least in the USSR). They just measured your belly.

Its just that it is interesting that there could be ethnic differences in gestation periods, as these studies would suggest. If this is indeed true, then that would explain why everyone back in my mom's day in USSR counted gestation weeks differently.