Due dates calculation

[Ivanhoe]

I have a doubt:

THe ob i saw at the beginning of this pregnancy (Dr 1) used the following formula to calculate due dates: Last menstrual period + 9 months + 7 days

The midwife (as well as her back up Ob) that will help with this birth does the following: LMP + 9months + 10 days

The backup midwife goes with the following maths: LMP + 9moths + 14 days


So? Who is right? I know due dates does not mean anything, etc, but today is my due date according to Dr. 1...and I'm starting to obsess.

[maisiedotes]

I have wanted to know the same thing, because every website I have been on and every one of those little wheel things the doctor's office has have said something different. I have no idea.

[Elena'sMami]

I believe the wheel that OBs use assumes that your normal cycle is 28 days, which as we know is an average, but the range of days in a cycle is much greater. I've read that if your cycle is longer or shorter than average, that can affect your EDD. Let's say your LMP was Jan 1...averages assume fertilization at 14 days later, or Jan. 15. But if your cycle is 36 days long, then fertilization would occur 18 days later, on Jan. 19, which would push your EDD back 4 days. If your cycle is shorter than 28 days, it could push your EDD up. In addition, your ovulation might have been off by 1 - several days for that month, which would affect the date of conception, as well. At least, that's what I understood. Sorry I don't have the article to reference - hope I haven't mangled the explanation.

Anyway, probably your OB used the standard calculation, and your midwives are going off of more exact information, such as the normal length of your enstrual cycle or information you provided about when you DTD. My midwife did an initial calculation of EDD based off of my LMP. He said, "So, conception probably occurred on Feb. 8." I knew for a fact that it had not occurred on Feb. 8, but rather Feb. 11 or later, so I got him to change my EDD from Oct. 31 to Nov. 3. Now that I'm at almost 42 weeks, I'm kind of sorry I didn't push him to make it Nov. 4 or 5, since fertilization doesn't always happen on the day of, um, you know.

Hope that helps.

Rebekah

[Ivanhoe]

Thanks for the explanation Rebekah. I understand the lenght of menstrual cycle factor. However, I don't understand why the ob's wheels assumes one can conceive on day 7 of a month. I mean, that's almost impossible! At least, that, the 7 days adding to the nine months has nothing to do with conception date...

I'm going to bed, hope all of you have a good night!