Why Does Everything Go By Your Last Period

8 Replies
sm - October 14

I went in for my 20 week u/s and the dates matched perfect, they said I concieved around june 1st. I jst dont understand why everything is off of your last period!! It drives me nuts. If i got preg a few adys before my last period would it had changed the dates on my u/s?? Just wondering..I'm confused!! any help would be great..thanks


... - October 14

They go by your last period, because that is a definate date they know for sure. The day you start AF is the 1st day of your cycle. For most couples it's almost impossible to pinpoint the day of conception, so instead of trying to figure out the exact day you conceived, they just go by your first cycle day. If you have an ultrasound and it shows you are farther along or not as far along, sometimes they will change your due date. My baby measured 2 days behind for how far along I was by my last period, but since it's only 2 days, she didn't change my due date. I hope that makes sense.


~m~ - October 14

That's just their rule of thumb, since ovulation "normally" comes 2 weeks later. It's kindof strange I know, especially since not all women are on "normal" cycles. But that's how they do it. And just an fyi for ya, the date the ultrasound says may not match exactly, but the deviation from the actual due date is only + or - 5 days. So it's pretty close. Good luck!


~m~ - October 14

to ... - I just read your post. My us was just off by 2 days also. Funny how accurate modern technology can be!!


kr - October 14

I know what you mean. I remember my date of conception (my husband's birthday), but my U/S says otherwise. I think the thing to keep in mind is that the baby could come any time (a week or 2) around your due date.


MJM - October 14

I know what you all mean. I know when I conceived also and my Dr. thinks that it is 7 days thereafter. I just have to play it by ear and when my "due date" draws near I will just be expecting our son at anytime.


~m~ - October 14

Well, there goes my 5 day rule. I sure thought that was the degree of error. But it looks like it can be at least 7 days. MJM, I know what you mean. Sometimes we have to go by our gut!!


Charity - October 14

My first u/s said I'm due 01/17/05 the dr. said 01/25/05. But now they are saying 01/23/05. Oh well I guess I'll be due when she get here. 1 in every 15 have their baby on the due date. Right?


Wondering - October 15

But what happen is you are at 42 weeks and your doctor has to induce you. Does that mean that your due date was wrong or that you are going over your due date and a baby can actually past it? Anybody ever wonder what would happen if doctors did not induce.



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