Now, those drugs are Category C, meaning, they have no known effects on the fetus but also meaning they have not done adequate studies because what Dr. in their right mind wouild want to pump pregnant women full of narcotics right?
Anyway, the one thing I can tell you is that it's far more dangerous for the mother to be in severe pain because pain elevates blood pressure and adds stress to an already stressful situation so they would rather the mother be as comfortable as possible.  But as with all medications the benefits for taking these tyepes of drugs must outweigh the risks.
That being aid, as far as I know the only real thing most doctors are concerned with is the possible addiction and withdrawal of the baby when it's born so they urge women not to use these medications too close to delivery if possible.  However, the dr can simply monitor the baby and take care of the addiction and withdrawal if they are aware of the situation.  I was fortunate.  My daughter was fine.  They DO know what to look for as far as drug withdrawal though so that's another reason to let yuor dr in on it.
As far as methadone.... well, I don't have that much info on it.  I do know that most experts would rather the mother stay on it than stop c/t during pregnancy because it can put the baby in grave danger and fetal distress.
I wish you luck.  Try not to beat yourself up about it but DO GET TO THE DR ASAP.
Here is a link to a helpful website on methadone and pregnancy:

http://www.exchangesupplies.org/publications/methadonehbk/pregnancy.html

I just found this from that site:
If you think you might be pregnant, don

Hey Ladies -

Ladies, I'll try to keep this short....as I'm still feeling REALLY bad - (much like you GB - have reinjured myself - and am taking more meds than I was a month ago - but not more than prescribed by the doc!)

Sereh - here is the website that I can refer you to.  I know nothing about methadone -but Sharon, the moderator on this site is awesome.  EMAIL HER!  She will email you back - she is REALLY helpful - especially when it comes to Moms on methadone.  She can refer you to the right folks!  Also, the moms on that site have been through what you have been through -and can help you trememdously!  But let me echo what has been said - you need to see a doc or a support clinic that will get you through this pregnancy - and get you to a healthy, happy baby!

http://www.methadonesupport.org/Pregnancy.html

Girly - THANK YOU for posting your story.  As I am now 5 days away from the big day - I'm gettting more and more nervous about the baby.  I am not going over 30mg of hydrocodone - but I have increased from 10mg daily - and I'm petrified that the baby is going to have issues.
The docs know about my increase - as when I went in after bumping my head - I was sent to see the neuro right away.  Your story helps quell some of my fears - but I just can't help it.  

I feel for you - I know how well you have done fighting the pills - and then an accident happens.  I told someone else on the board that after all of this is over - and I have another surgery - I will be stronger than I was b/c of this board - and will be able to fight the pills.  I WILL use as needed for pain - and then have my hubby or someone who is my advocate keep the meds.  Also - my doc will know not to prescribe them like water to me!  I want everyone involved - so I set myself up for success.  

Girly - Hang in there - once again - THANK YOU for your story - it gives me hope -

Sereh - check out that site - email her TODAY - and you'll be on your way!

To all on the board - won't be checking in for awhile - but ALL of you are in my thoughts and prayers - as we are in this together - and your strength gives me such strength!

me