Since they go by 28 days?

Question

Since most doctors go by 28 days to figure out when you are due such as your LMP, how will you ever know how exactly far along you are? Or will you ever?
According to my ultrasound my LMP was 7/19/06. My ultrasound matched my LMP for the 20 week ultrasound. Well I have NEVER had a 28 day period. So they gave me a due date of 4/25/07. That's fine with me, but if I ovulated later could the baby not be as far along?
I know you really aren't pregnant until roughly about 2 weeks after your period, so if you baby comes at 38 weeks does that mean something more than 40 weeks?
Sorry for all the questions, I've just been wondering.

Answer 1

calculating a due date by using your menstrual cycle as a guide just gives you a rough estimate at the beginning. the doctor uses the average 28 day cycle to estimate when you conceived - ie most women are most fertile about 14 days before their period starts. so the doc will take the date of your last period, add a fortnight, and guestimate that was when conception took place. the due date will be 40 weeks from the calculated conception date, not from your last period.
if you have a longer cycle, the date is likely to be later, as you won;t have ovulated until slightly later. if you have a shorter cycle, then you'll have ovulated slightly earlier than the doc calculated, and so you'll be a bit further on than they've guessed.
however, conception can take place 2 or 3 days either side of your *most fertile* day, and not all women follow the perfect 28 day cycle, so it's not particularly reliable.

the ultrasound gives you a fairly accurate picture of the baby's development. the doc will compare this to the average development of a foetus at different stages to give a *better* idea of how far along you are.

but it is always just a guide. different babies can develop at slightly different rates.
and babies can come early, or late...

anything 2 weeks either side of your due date is *normal*, and considered *full term*.

Answer 2

I honestly didn't understand your questions, but I can try to explain it to you as I know it. The "9 months" that you are pregnant is the same as the 40 weeks and the 10 lunar months. Each week is 7 days, and there are 4 weeks in a month. We know that doesn't add up to the calendar months, which is why it comes out to be 9 months. The ultrasounds are on the lunar schedule, too, and your doctor will relate everything to you in this scale. The measurements are all relative. Your due date is always "give or take" a little bit. If you had a period on 7/19, then you would EXPECT your next period at approximately 8/19 but you ovulated and conceived at some time in between. it's really impossible to tell exactly when, so the median is chosen as your "conception date" and you go 40 weeks from then. Although your projected due date may be off a little bit, once they start measuring the baby by the head, leg and total length, they can tell how old your baby is by its growth. The ultrasounds aren't exact depending on your baby's position but they are accurate enough to rely on for gestational age. but remember that no matter how far along you are, you could always go into labor early! (or you could be overdue)... don't worry about it, though. all of these things won't matter in the long run.

Answer 3

Its an inexact science. Most Dr's will go by CRL determined through Ultrasounds. However, those are accurate within a week.

However, even if your cycle was normally 30 or 35 days, we're only talking about a weeks difference. The 28 day assumption assumes you ovulate on day 14, and have a LP of 14 days. You could O later or earlier but your LP will almost always be the same (varies by person) and never more than 17 days.

In any case, to answer your questions: 40 weeks is from LMP. If your O date doesn't fit into the 28 day model then its possible that your edd could be a week off. I wouldn't sweat it if I were you since a very VERY small percentage of babies actually come on their due dates.

Answer 4

Usually, they use the 28 days as an estimate, in the doctor's office before you get the ultrasound. When you get the ultrasound, they are looking at the size and development of the baby to see how far along the baby is regardless of when your period was. This is the more accurate estimate. Because there is so much going on with the baby in the first trimester, they can get a pretty good idea. If you have had an ultrasound of the baby then you are pretty close. There is no way really to tell the exact date of conception but they can usually get it within a few days.

Answer 5

I'm like you, I've always had a 5 week cycle, and so my ultrasound measured things about a week smaller than my due date. The doc still didn't change the due date - but I pretty much figure this baby is going to come when she's good and ready to come (could be earlier, could be later) and nothing any doctor says is going to be able to change that.

Just take it as it comes.

Answer 6

Even though ultrasounds are much more accurate than they were when our moms were pregnant with us, the calculated due dates can still be off by two weeks even if you are a perfect 28-day girl. So, no, you can't tell exactly how far along you are.

Answer 7

I had an ultrasound to determine how far along I am. I had aa irregular cycle, so trying to tell by my LMP was not very accurate.

Answer 8

at an ultra sound scan they get the date from the size and development of the feotus, so this is likely to be more accurate than your LMP dates, especially as you dont have a regular cycle. So go with the dates they gave you at your scan and good luck :)

Answer 9

unfortuantly i cannot help you. i just found out 2 weeks ago, that i am exectingm y first baby. YAY!!! i myself have never had a 28 day cycle. i am usually anywhere 30-35 days. every where ive looked says im 7 weeks, but im thinkings its more like 6 since i dont have the "typical" cycle.

sorry i coudnt help, but i was wondering how my doc. would give me an edd.

Answer 10

I don't think it matters because baby comes when baby comes. so stop worrying and go of the date they have given you then wait and see.