Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI) began in 1977 and uses magnetism, radio waves, and a computer to produce images either as individual slices or reconstructed to give three dimensional (3D) views of specific anatomical regions or structures.
A strong magnetic field (up to 1.5 to 4 Tesla) is generated in the machine through which the body is passed (the centre of the "donut ring" seen in the above image). The earth's natural magnetic field is about 0.5 Gauss compared to 15,000 Gauss (1.5 Tesla) in the MRI.
MRI can be used in fetuses at 18 weeks gestational age or later and has been used mainly in brain and spinal diagnosis, and has also been used to investigate other abnormalities of pregnancy. (More? BrighamRAD Twin Gestation with Complete Hydatidiform Mole)
See also the related page MRI Movies which discusses MRI as a tool to study normal development.