Placental growth factor (PlGF) is an angiogenic factor identified in the maternal circulation, and a key biomarker for the diagnosis and management of placental disorders. Furthermore, enhancing the PlGF pathway is regarded as a promising therapy for preeclampsia. The source of PlGF is still controversial with some believing it to be placental in origin while others refute this. To explore the source of PlGF, we undertook a prospective study enrolling normal pregnant women undergoing elective caesarean section. The level of PlGF was estimated in 17 paired serum samples from the uterine vein (ipsilateral or contralateral to the placental insertion) during caesarean section and from a peripheral vein on the same day and second day post-partum. PlGF levels were higher in the uterine than in the peripheral vein with a median difference of 52.2 (IQR 20.1-85.8) pg/mL p = 0.0006. The difference when the sampled uterine vein was ipsilateral to the placenta was 54.8 (IQR 37.1-88.4) pg/mL (n = 11) and 23.7 (IQR -11; 70.5) pg/mL (n = 6) when the sample was contralateral. Moreover, PlGF levels fell by 83% on day 1-2 post-partum. Our findings strongly support the primary source of PlGF to be placental. These findings will be of value in designing target therapies such as PlGF overexpression, to cure placental disorders during pregnancy.