Lactational ammenorrhea occurs in a women after delivery while she is breastfeeding
the baby.Return of menstural cycles varies for each woman and may range between 2 months to 2 years depending upon how actively the mother is lactating (breast feeding). For the person to concieve the menstural cycles should be ovulatory. Generally, following delivery the first menstural cycle is always anovulatory. But subsequent cycles are always ovulatory. Since your wife's periods are not the first one after delivery it should be considerd as ovulatory cycle (fertile period) & hence you cannot rule out pregnancy
. But, do not worry. Because the luteal phase in the initial menstural cycles after delivery is usually short to sustain pregnancy.