Ultrasound for Dating the Pregnancy
Ultrasound can be used to date a pregnancy. This is very useful if you are unsure of your dates, have extremely irregular or extra-long cycles, or have a surprise pregnancy while not monitoring your cycles. However, as with other uses, ultrasound for dating can also be a mixed blessing.
To determine the gestational age of the baby, ultrasound techs measure various parts of the baby and compare it to a fetal growth chart. Early in pregnancy they measure the "crown-rump length" (top of baby's head to the end of its bottom). Later in pregnancy, they measure the length of a leg bone (femur length) and from one side of the baby's head to the other (biparietal diameter, or BPD). For fetal weight estimates, they add a measurement of the baby's abdominal circumference. These are the most common formulas used, but sometimes other measurements are done too.
When babies are first conceived, their growth is fairly uniform in the first few weeks, so gestational age dating in the first trimester is fairly accurate. As babies continue to grow, however, biological variability takes over, and babies begin to grow at different rates. This means that gestational age dating is less accurate as the pregnancy continues. In the second trimester, it is fairly accurate, but by the third trimester it is quite inaccurate.
Most ultrasound resources state that ultrasound for determining gestational age in the first trimester is accurate to within about a week. Measurements in the second trimester are supposed to be accurate to within about two weeks. By the third trimester, the accuracy rate has dropped significantly and can be off by as much as three weeks.
Therefore, if your cycle is very irregular and you need to get the most accurate dating possible, it is best to have an ultrasound as soon as possible after you know you are pregnant. In this case, the possible risks of exposure to ultrasound in the first trimester are probably outweighed by the pressing need to have an accurate due date.
Some women, though, don't realize that they are pregnant for a while and may not still be in the first trimester. Even though the ultrasound dating will be less accurate, it is still worthwhile to have it done in the second trimester so that the woman is not induced prematurely for "post-dates" pregnancy. Research is clear that ultrasound is very useful at reducing the number of "post-term inductions" in this instance. (For references, see the website at https://cpmcnet.columbia.edu/texts/gcps/gcps0046.html.)
However, if you are sure about your dates, ultrasound dating is not very useful. On occasion, women sure of their pregnancy dates through fertility charting etc. have been forced to move up their due dates because of ultrasound results. Often this results in babies being induced or delivered prematurely, which potentially has major health consequences.
Therefore, be very leery of a doctor moving up your due dates based on ultrasound, especially an ultrasound done later in pregnancy. Remember that the sizes used to calculate maturity and due dates are based on averages, so if your baby tends to run slightly larger than average (as some large women's babies do) then the baby's estimated age may be off.
Also, the measurements used to calculate the baby's age are not always easy to do, and small miscalculations can introduce significant errors into the dating process. For example, one resource points out that a baby's head is not perfectly spherical but more of an ellipsoid, and so the measurements from one place to another are going to differ slightly. Even this slight difference in measurement can cause significant differences in datings at times. In addition, Dr. Marjorie Greenfield at
www.drspock.com cautions:
Due dates by ultrasound are not that exact. Remember that we are measuring a flat image of a three dimensional fetus, and then estimating how far along the pregnancy is by the millimeters of the leg bone or of a line drawn across the head. There is a lot of room for being a few millimeters high or low on all of these measurements, not to mention that babies of the same gestational age can be slightly different sizes.
Ultrasounds in the last trimester to predict a baby's age/due date are particularly inaccurate. Unless there are really extenuating medical circumstances, do NOT let a doctor move up your due date based on a third-trimester ultrasound. There have been big moms who have had their babies delivered prematurely because an ultrasound scan inaccurately estimated the baby's age and the doctor moved up the baby's due date. Prematurity can cause big problems, so this is not a decision to make lightly. Unless there are very unusual circumstances, ultrasounds in the last trimester should not be used to change due dates.
well i would plan for your HB then anyway?? My MW pretty much said the same to me, that she doesnt think i'll make it to hospital (mine was longer than yours though 
) 
is going to come fast i would just plan for it 
Obv if it was too early then you want to get in but if you know your dates and are that confident about them... nobody is going to knowingly put their baby at risk
and my childrens father has to get here from 20 minutes away to look after the children before I can go anywhere!
