Examined the effect that the birth of a sibling had on the attachment relationship between a mother and her firstborn. The Strange Situation Procedure was used to assess quality of attachment between 20 mother-infant pairs (mean infant age = 16.3 months) during the last-trimester of the second pregnancy and at 6-10 weeks following birth. These results were compared with those obtained from a control group of 20 mother-infant pairs (mean infant age = 17.6 months) where the mothers were not pregnant.

At the second testing period, 19 changes of attachment (global or subgroup classification) were observed in the pregnant group compared to 7 in the control group. In both groups, the changes in attachment style were about evenly split between changes towards greater security and changes towards greater insecurity of attachment.

Overall, the authors conclude that pregnancy and birth of a second child leads to instability in the mother-infant attachment relationship but not to specific changes towards increased security or insecurity. Rather, the nature of the change depends on external circumstances such as temperament of the new infant, relative temperament of first- and secondborn infants, mother's well-being following birth etc.

The authors note that there may also be changes in the interactions between firstborn and father, in line with previous research indicating the increased importance of the father's role during the second year of life.