A puppy fetus is an embryo that is beginning to resemble a puppy in form. This happens at about 28 days after conception. The developing fetuses can usually be palpated by human hands in an effort to confirm a pregnancy from 25 to 26 days after breeding. Calcification of the fetal bones takes place at about 45 days of age. This makes the visualization of the fetal skeletons possible with the aid of radiographs (x-rays). Ultrasound techniques are also beneficial to confirm pregnancies. Pregnancy lasts about 63 days in the dog, although it may be as short as 56 days and as long as 72 days. It is during the pregnancy (or gestation period) that the developing puppy grows inside the mother's uterus and derives nutrients and oxygen directly from her blood. This exchange occurs via the umbilical vein and artery which connect the mother to the fetus.

Since the fetus is connected to the mother's blood supply, infections, poisons, etc., which affect the mother, may be detrimental to the developing fetus as well. Occasionally, the fetus will become ill or even die while still within the mother's uterus (womb). If this happens early in development, the fetus may simply be absorbed by the mother's tissues, eliminating any evidence or knowledge of the pregnancy. Fetal deaths, which occur later in gestation may result in abortions, the expelling of the dead fetus, and/or uterine infections, as a result of decaying fetal tissues. There are several common disorders which may retard fetal development and/or cause death.

Developmental abnormalities generally include disorders relating to abnormal fetal development. This includes instances where the fetus fails to develop as expected. Examples would be cases where the fetus fails to develop certain organs or appendages. Puppies are occasionally encountered which have a lack of brain tissue. Similarly, other organs such as kidneys may be partially or totally absent. Usually, these malformed fetuses die before or at the time of birth. They may be aborted or born dead (stillborn). There are no specific symptoms, risks, or treatments in these instances, as most cases are different in their character. The result is fetal death.