Babies while in the womb are surrounded by fluids, so they cannot breathe. Do they get oxygen through the mother's umbilical cord?
Are we kind of amphibious before coming to the world?
The foetus gets oxygen through the mother's umbilical cord:
The fetus receives its oxygen supply across the placenta, to which oxygen is delivered by maternal blood. The amount of oxygen delivered to the placental site is determined by the magnitude of blood flow and the oxygen content of uterine arterial blood. Transfer of oxygen from maternal to fetal blood is influenced by the diffusion gradient for oxygen between the maternal uterine blood and fetal umbilical blood.
For further information about the umbilical cord and the placenta, see this Wikipedia image.
No. Although foetuses have pharyngeal arches the "oxygen supply line" is the umbilical cord. A foetus can not live outside the womb due to a lack of oxygen supply (the lungs are not yet working) and general supply of vital molecules. This is why they can not be considered amphibians.
The word "amphibian" is derived from the Ancient Greek term ἀμφίβιος (amphíbios), which means "both kinds of life", ἀμφί meaning "of both kinds" and βιος meaning "life". The term was initially used as a general adjective for animals that could live on land or in water, including seals and otters.