Hi Den,
Since Methadone has been around for so long there are far more studies done with pregnant women and Methadone.
Something that you may be interested in reading is the TIP 43. Chapter 13 page 211 (Medication Assisted Treatment for Opioid Addiction in Opioid Treatment Programs)
Here is the link for the publication. You can read it here:
http://www.ncbi.nlm.nih.gov/books/NBK64164/
Or you can download it to your computer for future reference from here.
http://store.samhsa.gov/product/TIP-43-Medication-Assisted-Treatment-for-Opioid-Addiction-in-Opioid-Treatment-Programs/SMA08-4214
If your looking for particular studies here is one study of Buprenorphine during pregnancy:
http://www.medscape.com/viewarticle/739191
Study, Methadone VS. Buprenorphine
http://www.ncbi.nlm.nih.gov/pubmed/21142534
another study
http://www.ncbi.nlm.nih.gov/pubmed/15943939