I suggest the following answer on the basis of the Gemara above Yevamos 42a, which asks that when 3 full months have passed after the death of her husband we should check the widow to see if she is pregnant. Rashi DH v'Ki writes that we should check her nipples ("Dadeha") as is the way of expecting mothers (i.e. the milk starts developing there).

It would seem that there is no difference between a woman pregnant with twins and between a woman pregnant with just one baby in regards to the development of Dadeha, because the latter does not depend on the weight of the embryo, but rather on the development of the body in order to prepare it later on to feed the baby with milk after it is born. (In this respect the Dadeha test is different to the test mentioned immediately afterwards in the Gemara - to see how she walks - which Rashi explains to mean that we observe the depth of her footprints in the soil. It is possible that twins are slightly heavier than a single child and therefore the footprints would be deeper).

I am not suggesting that a Dadeha test was actually performed on Tamar. Rather the Gemara in Nidah 8b means to say that 3 months after she was with Yehudah it was known that she was pregnant and that if one would have performed a Dadeha test 3 months after conception it would certainly have been positive.

The Rashash and Keren Orah to Yevamos 42a write that the Dadeha test is 100% effective at 3 months, whilst the walking test is less foolproof at the same stage. See also Teshuvas Rebbi Akiva Eiger #89 who also discusses simply observing the woman to see if one can notice that she is pregnant. This also is not always effective at 3 months. (See also Chazon Ish EH #134 to Yevamos 42a.) However, since it is always possible after 3 months to determine whether or not she is expecting, through Dadeha, this proves that it is irrelevant whether or not she is expecting twins, because one can assume that the development of the mother's milk is identical whether she is bearing twins or just one baby.

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D. Bloom

Rashi (Yevamos 42a DH v'Chi) writes that we can determine whether a woman is pregnant after three months by checking her for physical changes relating to lactation. This should not change with the number of babies she is expecting.

This test is different from the test mentioned immediately afterwards in the Gemara - to see how she walks - which Rashi explains to mean that we observe the depth of the woman's footprints in the soil. The Rashash and Keren Orah to Yevamos 42a write that the lactation test is 100% effective at 3 months, whilst the walking test is less foolproof at the same stage; see also Teshuvas Rebbi Akiva Eiger #89 and Chazon Ish EH #134 to Yevamos 42a.)

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D. Bloom