This is a routine ultrasound scan performed at 35 weeks of gestation. These are pictures of the leg (the rest of the fetus is normal).

The diagnosis was not established until the fetus was delivered and a post-natal biopsy revealed a spindle cell tumor that was confirmed to be a fibrosarcoma by immunohistochemistry. The translocation t (12 ;15) ( p13 ;q25) was absent. The antenatal images demonstrate a vascular pedicle leading to the mass:

Postnatal photograph: 

In the immediate post partum, the tumor grew massively requiring partial resection. The child did not have an amputation and was also treated by chemotherapy. The following are the histopathological photographs:

Currently the child is doing well, walks with a limp and the calf is supple without palpable tumor.

Discussion: Congenital fibrosarcoma is a pediatric spindle-cell tumor of soft tissues that usually presents before the age of 2 years. Although these tumors display histological features of malignancy and frequently recur, they have a relatively good prognosis and only rarely metastasize. Soft tissues tumors account for 7% of the malignant tumors in childhood and fibro- and myoblastic tumors account for 12% of the soft tissues tumors in childhood.
A recent study about the follow-up of congenital fibrosarcoma at CHU NECKER – Enfants Malades (Paris) by Dr J-C FOURNET revealed these features of the mass:
Age : newborn to 2 years old with neonatal diagnosis in 80% of the cases.
Sex : M > F ( x 1,7) 
Localization : Head and neck 19%, trunk 31%, abdomen and retroperitoneum 8%, limbs 42 %
Recurrence : 5-50%
Metastasis : 0-25%
Mortality rate : 23%

A prenatal diagnosis is impossible. Diagnosis can only be established by biopsy and t(12 ;15) (p13 ;q25 ) translocation when found, gives a better prognosis.