Kate tells her husband Pete that the child he thought was theirs is not at all Pete's child (the audience knows it must be Phil's, Kate's lover).
The amount of time needed (expecially in 1929) for a woman to know she is (or could be) pregnant is approximately one month. Pete returns home, after a quite long absence; he and Kate marry. In the movie is not specified how long after his return they marry. Let's assume they marry the very same day of his return. Later, Kate tells Pete that she is pregnant. Again, we don't know how long after their marriage she tells him that; in any case, she shows no exterior sign of pregnancy when she tells that. Had she and Phil had a sexual intercourse (coitus), which made her pregnant, say, the day before Kate's marriage with Pete, the birth of her baby, if conceived with Phil, would have happened not after eight months of marriage, maximum. PRETERM BIRTH, if the baby was assumed to be conceived with Pete. And Pete must comply with that, if the movie has to keep going.
Otherwise: if the child was conceived by Kate with Philip AFTER her marriage with Pete (nothing, in the movies, points to that end, and nothing points against it), in that case (expecially in 1929), nobody could tell who was the baby's father, and we will have had no movie.
The amount of time needed (expecially in 1929) for a woman to know she is (or could be) pregnant is approximately one month. Pete returns home, after a quite long absence; he and Kate marry. In the movie is not specified how long after his return they marry. Let's assume they marry the very same day of his return. Later, Kate tells Pete that she is pregnant. Again, we don't know how long after their marriage she tells him that; in any case, she shows no exterior sign of pregnancy when she tells that. Had she and Phil had a sexual intercourse (coitus), which made her pregnant, say, the day before Kate's marriage with Pete, the birth of her baby, if conceived with Phil, would have happened not after eight months of marriage, maximum. PRETERM BIRTH, if the baby was assumed to be conceived with Pete. And Pete must comply with that, if the movie has to keep going.
Otherwise: if the child was conceived by Kate with Philip AFTER her marriage with Pete (nothing, in the movies, points to that end, and nothing points against it), in that case (expecially in 1929), nobody could tell who was the baby's father, and we will have had no movie.