As with most opioid medications there just isn't that much information. One site said 

There are limited data available concerning the use of dihydrocodeine in human pregnancy.  These data do not indicate an increased risk of fetal toxicity.  Use of any opioid during pregnancy, particularly around the time of delivery, confers a risk of neonatal respiratory depression.  Prolonged use of opioids throughout pregnancy could result in neonatal withdrawal.

There's obviously  some grey area. There are some risks and given that it makes sense to take as low a dose as possible. Hopefully your gynecologist has experience with opioid use during pregnancy and she/he can give you guidance in this area. Since few studies examine this area experienced physicians are probably your best bet.