Need help with baby waist and thigh measurements

I'm making a couple of baby dresses as gifts, but I have a problem with the pattern (Butterick 3405)--maybe someone can help?
The bloomers on the pattern have elastic at the waist and thighs but the pattern doesn't say what length to use, it says to measure the baby.  Since I don't have one (a baby that is) I can't.  I'm making size M (18-21 lbs, 27-28 in).

Any help you can offer would be greatly appreciated!  thanks.

I have a chubby 18 pounder myself, so maybe you could just use her measurements?

For the waist  I use 16-17" of elastic and for the thighs I use about 10". The softer your elastic, the less you can use since it stretches more. My baby has thunder thighs, so you might want to take that into consideration  .

Thank you Jorja!  That helps so much--I can finally finish the dresses and send them off before the girls are too big to wear them!

Hi does anyone out there have the waiste and thigh measurements for all the bloomer sizes??  My pattern did not have them either!!  Thanks in advance

I could give you some measurements I've used, but babies are hard to fit because you never know what their size will be.  I guess it is best to err on the "too big" side, since eventually they will grow!  Another good rule of thumb is that if you can find a chest size, then usually the waist size will be close to that.  And if all else fails, do what I do and have a mini measuring tape on your key ring and measure clothes at the store!  Here goes:
0-3 mo  waist: 13-16 inches  thigh:  5-10 inches
3-6 mo  waist:  15-17 in         thigh:  10-13 in
6-12 mo waist:  16-19 in        thigh:  11-14 in

I hope this helps some!  I know that it is only a "range" of measurements, but it has worked for me, unless I am extremely lucky. 
Happy Sewing, Lauranie

Thank you Lauranie. I am in the middle of making bloomers for a couple of babies and was wondering the about the waists. Your measurements seem to be about the same as ready to wear waistbands.