I would not worry at this time, but obviously you will want to watch closely. Usually, language development in twins lags behind singletons. One reason for this is that sometimes twins develop a “private language” they use only with one another. Usually the adults can’t understand a word of it, though sometimes they can figure out what specific objects are being called. But private language is fairly rare, and it doesn’t sound as though your girls are demonstrating it.
Another reason for the lag is that, no matter how hard parents try, it is difficult to give two babies as much one-on-one attention as single children receive. I well know the validity of that statement, as I am the mother of twins. Find every opportunity to talk and read to each baby individually. If you can read a bedtime story to one and their father can read to the other, that would help. And encourage the other people involved in their care (grandparents, nanny) to react to them as individuals. People tend to fall into a “groupthink” pattern when you have twins, even though the group has only two individuals in it. They will ask you, “How are the twins?” as though the answer should be the same for Katherine as for Abigail. Try to encourage everyone to see each girl as a distinct individual.
I have to disagree with you on one point, and I mention this only because it may be relevant to your concern. You say that Abigail and Katherine “are not in day care.” Even though they might not be enrolled in a day care center or in a family day care home, they are definitely in day care. Therefore, an important aspect of my answer to your question relates to the quality of the learning experiences they have with their nanny. She may spend all day watching TV, with your girls propped up on the sofa. The toys you have bought to enhance their development might be left on the shelf. There are many kinds of day care, with nanny care being available only to a few families with higher incomes. But it is definitely day care and, like all other models, it can vary widely in quality. Check it out.
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