Telling Dr.'s your date of you last period, how it affects due date
If it was out by more than that then it may become obvious as your baby grows that it is noticeably bigger or smaller than your dates suggest. This will probably prompt your Dr to suggest growth scans and may change their advice regarding induction etc. they'll probably also clarify a few times that you are certain of the dates.
I assume you've intentionally told them an incorrect date for some reason but, if you change your mind you can always just say you made a mistake and your actual LMP is...
ETA - it also depends on how closely your babies adhere to the typical foetal growth patterns. If your babies tend to bigger or smaller it may be more or less obvious depending on whether your HCPs think you are further or not as far along as you actually are.
I usually have regular periods / cycles but the last one in which I got pregnant, I swore that I ovulated a week after my period rather than my usual two weeks. My midwives set my due date based on my last period, but I suspected I was a week ahead of how far along they said I was. Today I had my first ultrasound and while the midwives put me at 9 weeks, the ultrasound tech could tell right away I was at least 10... so it's not hard for them to figure out accurate due dates BUT they also are quick to acknowledge that everyone is different and that there is a range of "normal" so, especially as you go further along, depending on how close your "due date" and "actual due date" is, it will be harder to tell if it is incorrect or not.
Do you know your ovulation date? I ovulated a week later than usual this pregnancy so I calculated my due date based on conception date and asked my HCPs to use that date. It wasn't a problem. Or, if you don't know, you could tell them your LMP but also that you don't think your ovulation was Day 14 and request a dating scan (depending how far along you are). The two weeks assumes a 28 day cycle with ovulation on day 14.
If you give them a LMP date they tend to calculate due date based on the assumption that you ovulated on day 14 of your cycle and have 28 day cycles :) My cycles never are, so I always tell my midwife my LMP and I also tell her ovulation date if I know it. I prefer to use a formula calculating based on ovulation date, that has been more accurate for me.
I know I ovulated on day 18 this cycle which is later than day 14 and I didn't want to hear anything about going "past due" so I let my midwife know I'd ovulated late (my first real cycle while breastfeeding)... if you have long cycles I would def let your doctor know this because it makes it more likely the due date based on LMP will be earlier than what your baby actually needs to cook :D
I never used pee sticks, so I'm not sure how they work or what exactly they are measuring...but, unless you ovulated way past day 14, then there isn't going to be much difference between determining the due date based on conception vs LMP. And those dates may be slightly different than what they determine from a "dating" ultrasound. It's not an exact science. For me, I was checking my cervix and I know that I ovulated right about the 10th of the month, which would put my DD at about June 3. According to an early ultrasound, they put my date at May30, which is 4 days different. For me, I won't make a big deal of it. It's mentioned in my charts that I was pretty sure of ovulation, so if they really want to induce me, I can argue to let it go another 4 days or so. But, due dates are estimations, so usually a couple of days isn't going to matter much one way or the other, unless you had a super long cycle that would change your dates by a week or more. I don't think it's known exactly how long it takes for conception to occur, and it can be different for different people, or in the same person in different pregnancies. On top of that, it's not entirely known what causes a woman to go into labor, so pregnancies can last for different amounts of time. I've heard that some places don't really give a specific due "date", but a due range of dates. If you are trying to determine the moment of conception, you would have to know when ovulation was and how recently you BD'd...it takes, what, 24 hours or so for sperm to travel up the tubes? So if you were BDing before ovulation, there may have been sperm already close, if you BD'd after ovulation, then conception may have taken a little longer. And anything in between. Make sense?
Nicole730, what exactly do u mean for insurance reasons? Is that what you mean Katelove about induction dates? Like after week 41/42? Thanks for the advice about changing my mind. Just worried about how to make it the most accurate. What about if the LMP is the normal 2 weeks before ovulation? Could there still be a mistake, or is that more "safe", even though ovulation could be slower or faster, the extra two weeks is supposed to make room for that? I think I am wondering about what Nattery is saying about the different ranges of normal and what would happen in that kind of situation where theres two dates, one yours and one their guess by size measurements??
I fudged mine because my insurance that covered pregnancy started July 10th. We got pregnant after that, but the first day of my last period was before the tenth. Then my prenancy wouldn't have been covered because it would have been officially started before my insurance start date. It ended up working out in my favor because I had higher bps and protein in my urine. they didn't want me to go to 40 weeks, which was actually 41 weeks. baby was born ar 40w1d by my calendar, but 39w1d on theirs.
I gave them the date of my last period and the due date they gave me matched up to the due date I calculated from date of conception (which I just happened to know, because I knew when I ovulated and there weren't too many dates it could have been right around that point). My ultrasound dates have always been similar to that date, she's usually measured 2-4 days bigger than my due date but that's really not much...if I had a care provider who got super worried about me getting near 41/42 weeks (although I would just try to avoid having that care provider in the first place...) I would fudge dates.
I'm not sure what you mean by "safe"...early ultrasounds are reasonably accurate and your date sounds like it makes sense. Again your EDD is an estimation, most women go into labor two weeks before or two weeks after that date....so if your O date was three weeks out, then LMP wouldn't be the best estimation, but it doesn't sound like that's the case for you. The reason why it would matter is that the placenta tends not to function well after 42 weeks, so the medical community may want to induce you before you get to that point. IF you O'd late, then you may not actually be 42 weeks. But again, being a full week off in an early ultrasound would be rare, combined with the fact that you tested pos for O before that. And a couple of days isn't going to make a big difference in terms of the 4 week span that women generally get as an estimated due date. The particular day is the average within that 4 week period. Something like 10% of women actually give birth on their actual due date. does that make sense?
My situation is like Nattery's--I knew exactly when my LMP began, and gave them that date, but when we did the first ultrasound, the little one was measuring a week bigger than that. It seems like I probably ovulated a week or so out from my period, rather than two weeks. The ultrasound techs are pretty good at their jobs, and they'll be able to see how far along you really are, or if the baby is bigger than it should be.