I don't have specific knowledge of a mom-fetus attachment scale, but in case you can't find one, there are several parental bonding questionnaires designed for the neonatal phase. I prefer the term "bonding" over "attachment" here because attachment means something different to attachment denizens (e.g., Sroufe, Waters). Here are 3 such scales. They weren't validated super extensively by their authors, but our group administered all three to a sample of moms and dads at infant age 1 month. They all seem to be reasonably unifactorial (despite claims of subscales) and they all seem to reflect the same thing (i.e., if you use the total scores of each measure as 3 indicators of a single bonding factor in structural equation modeling, model fit is good). I think that means you can probably pick an choose the one that seems to be the best fit for your project. Maybe modifying the items or instructions a little to reflect the fetal period would work?
Condon, J. T., & Corkindale, J. C. (1998). The assessment of parent-to-infant attachment: Development of a self-report questionnaire instrument. Journal of Reproductive and Infant Psychology, 16, 57-76. doi:10.1080/02646839808404558
Brockington, I. F., Fraser, C., & Wilson, D. (2006). The Postpartum Bonding Questionnaire: A validation. Archives of Women's Mental Health, 9, 233-242. doi:10.1007/s00737-006-0132-1
Taylor, A., Atkins, R., Kumar, R., Adams, D., & Glover, V. (2005). A new Mother-to-Infant Bonding Scale: Links with early maternal mood. Archives of Women’s Mental Health, 8, 45-51. doi: 10.1007/s00737-005-0074-z