My uterine height coincides to my conception date/time period but my last ultrasound suggested my baby was only 26 weeks. The ultrasound tech didn't know any of prenatal history so I am wondering if she could have been off with her conclusions.

I had an ultrasound preformed to confirm my pregnancy (I discovered very late on that I am pregnant.) About two weeks later at my first prenatal appointment my ob measured me at a fundal height that coincided to my conception date, however I had an ultrasound done after that and the ultrasound tech suggested that my baby's age was an entire month behind my fundal height/conception time. Neither of the ultrasound techs had any knowledge of my prenatal history. The second ultrasound, to me, is inaccurate because I didn't have sex during that time. I am wondering which is more accurate, ultrasound or fundal height measuring.

There is not test more accurate than the ultrasound. Fundal height can very for several reasons, one of which being amount of amniotic fluid, thicker than average uterus, full bladder, etc.

Between the two, ultrasound is routinely the more accurate. Fundal height can vary due to number of pregnancies, length of your torso, baby's position, amount of amniotic fluid at the time they measure, location of placenta, etc. Some midwives don't even do fundal height measurements for that very reason.

See what your doctor or midwife has to say about the scan results.

Remember, unless you know for certain when you conceived (not necessarily the day you had sex, but actual conception), and most women do not, dates can vary a fair amount.

Well I am more certain that I am either farther along or farther behind because I did not have sex at the time that would coincide with the ultrasound age. I actually did not know I was pregnant until a month ago prior to finding out I had been trying to lose weight. That definitely would affect baby's size, right? I am also fairly certain that there is as much room for error in ultrasound technology due to lump average sums as there is in fundal height measuring depending on all sorts of circumstances.

Correction to the above, I didn't have sex at all during the month I supposedly conceived due to the ultrasound tech's answer. This is why I am confused. I understand that sex/conception date are different up to two weeks (as far as fertilization/implantation) at most. This being the case, with a two week buffer, I cannot be 26 weeks. Either I am farther along or farther behind. If this is the case then ultrasound technology in my case is inaccurate.

Eudoxia, are you concerned about paternity? Have you been with two men and are concerned who the father is?

I am more concerned about my lifestyle during the time I didn't know I was pregnant. No matter when I got pregnant I still drank and under ate in a vicious cycle.

When do you think you became pregnant? Do you recall how far along you were when you first felt some fluttering? Ultrasounds can be off, as has been said, they tend to give the best estimate in most cases. Remember, it is an approximation to a degree and the scans are only as good as the tech who reads them.

Obviously, since there is a question as to just how far along you are, best thing would be to share your concerns with your doctor or midwife. Let them review the scan results, they may have another opinion, or they may suggest that another ultrasound be done in another two weeks or so.

As to your lifestyle before you realized you were pregnant, it may or may not result in a negative impact for baby, but odds are good that it won't. Many women will not know they are pregnant in the beginning, but make the appropriate changes once they find out that they are pregnant. In the vast majority of situations, baby is fine. This too would be something to speak to your care provider about to ease your concerns.

If I am as far along as I believe I am, I didn't consciously distinguish the baby's movement until the end of the second trimester. If I am as far along as the ultrasound says, I still didn't distinguish the baby's movement until the middle of the second trimester. Either way, enough of my pregnancy passed with me not knowing that I am concerned about possible damages. That's why I am asking about size and accuracy. If I am as far along as I think, then I have hindered my child's development in some way, even if it is just delayed. If I am closer to the ultrasound's results, than my child is huge for his age and that could still be due to something I've done.

I am most certain I became pregnant no latter than the second week of June 2010. If the ultrasound is correct it places me getting pregnant in July, which is absolutely impossible because I didn't have sex at all in July. Mid/Late August = next sex

I would discuss your concerns with your doctor so that they can repeat the scan if necessary and they will also likely schedule another one for a couple of weeks or so later to take note of any changes in terms of baby's size and development that they would expect to see

Ha, here's another reason why I feel ultrasounds aren't always so reliable.
I went to see a specialist because I was concerned about the affect my drinking and weight loss might have had on my child. The ultrasound tech at this appointment told me I am 29 1/2 weeks pregnant. How could I go from being 26w 1d to 29 1/2w in two weeks? Also, I have not seen the same doctor at each visit and they both measured very differently. The second doctor I saw measured me twice and (to my knowledge) not as accurately as the first doctor... so I'm sticking with my initial gut feeling because no on can give me a consistent answer. There's no way I could have conceived when they are saying I did. I am either a month farther along (with a small baby) or a month behind (with a big baby.)

No one said they are always so reliable, only that between fundal height measurements and ultrasounds, the scans are more accurate.

However, the later in a pregnancy you have the scans, the less accurate for dating purposes they become. Since you had yours later than the norm, this is likely why you are getting conflicting results. At the point were you are at, there is roughly a two to two and a half week window either side. This would fall right into place with what you were given at both scans. If they were concerned at all about baby's progress/size, they would let you know.

How far along do you feel you are likely to be? How does this measure up to the due date you were given? If you feel very confident in when you would have become pregnant, then go with that time frame. There is a several week window at the end of pregnancy that is a normal time frame to give birth in, so whether you are a little ahead or a little behind what they are thinking, you will likely still fall within the norm.