Early U/S and early HCG levels more accurate than later u/s?

Okay, I know it might seem strange that I'm asking this question now, but now I'm worried about my "real" due date in the event that I end up with a planned csection (both babies are breech).

Is an early u/s and early HCG levels a more accurate tool for dating the pregnancy than the u/s you get later?

According to my LMP I am 31w5d, but by my first u/s I am 31w1d. Also, my HCG levels at 30 days since LMP were 154 and they were 502 at 32 days since LMP. I know those numbers are really low, especially once we found out that we were having twins. But now my babies are measuring big, consistently about a week ahead of the due date established by my first u/s. Does that just mean my babies are big, not that the pregnancy is further along?

I know this seems like a crazy question, but I'm concerned that if I get a csection at what is 37 weeks by my LMP, that the babies might really be only like 36 weeks! So, which is more accurate, and would a few days to a week make a difference in their development.

 

U/S and HCG levels are really only accurate to date a pregnancy in the beginning. Even then, HCG levels are hit and miss since there is such a wide range of normal. The measurements vary much more in later pregnancy when it's harder to get accurate measurments so I wouldn't use that as an indication of when you're due.

Although every day inside is helpful, once you hit 36 weeks the risk for problems really goes down. If there are really only 36 weeks when you have your scheduled c-section they will likely do fine and not have any problems.

 

I'm not sure if this makes you feel better, but at the u/s place, they will tell you that their number is +/- 4 days, which would put you right on,

 

Early u/s are most accurate and HCG levels can be all over the place so they're not a good indicator. Some have really high (especially with twins) but some people have high levels and only carry 1.

I had my first u/s at 6w4d (when we found out) and I measured 6w1d. Since the difference was less than 5 days, they kept my regular EDD (6w4d). Later in my pregnancy they continued to measure ahead (2-3 weeks) and they were born above average singleton size ... so that was correct. 7lbs11oz and 7lbs10oz at 12 days early. They measured around 1 pound at 20 weeks, around 2lbs5oz and 2lbs4oz at 26 weeks, 4lbs6oz and 4lbs5oz at 30 weeks and 6lbs1oz and 6lbs15oz at 34 weeks (latter was known to be overestimated). So my u/s were pretty accurate.

HTH.

 

At each u/s the tech always reminds me that the weight percentiles can be off by up to 20%. SO u/s aren't perfect. A woman in my MOMs group had her twins last week and her babies were the size she had been told 2 weeks prior at an U/S so it was off.
Also, my understanding as the pp mentioned is that after 36 weeks they should do okay in terms of complications that arise with prematurity. Good luck!!