(1) The fetus is grasping his tumescent penis using an evident force. Foreskin is partially retracted. (2) In the next step, the foreskin is more retracted so it allowed us to locate the meatus clearly.
 
The human body is wired weird. Authentic sexual behavior commences in the second decade of life, but the wiring for sexual gratification has to be there—and indeed is there, as a paper authored by two researchers working in the Spanish city of Pontevedra, Vanesa Rodríguez Fernández and Carlos López Ramón y Cajal, claims to prove definitively—with visual evidence based on a volumetric rendering mode study.

The paper is called “In-Utero Gratification Behavior in Male Fetus” and has been accepted for publication in the journal Prenatal Diagnosis.

Here’s the abstract:
 

Fetal masturbation has been described previously once in-utero but only as a description of an action. Masturbation is well described in infancy and early childhood when they discover that this practice can give them pleasure. Our letter proves that it could begin in-utero as a ‘gratification behavior’. We have shown this pattern clearly using a volumetric rendering mode study.

 
The two figures shown here depict fetal gratification behavior using 3D ultrasounds.

The author writes:
 

We show “fetal gratification behavior” in a fetus in the 32nd week of gestation of a normal gestation. The fetus was grasping his tumescent penis with his hand and did it with such force that it shows us clearly the glans. Initially, the glans was covered by the foreskin and gradually the foreskin is retracted as we show in images the total sequence of the hand movements grasping the penis. ... We show this behavior using 4D live mode where the movements that go with this behavior are shown clearly. ... Moments later the foreskin covers partially the glans (Figure: number 4). This is a very clear sexual behavior “in utero” in the 32nd week of gestation. We understood this behavior as a “fetal gratification behavior” (fetal masturbation).

 

(3) The foreskin is totally retracted and the glans is very patent. (4) Finally, the hand movement returns the foreskin to cover the glans.
 
via Nerdcore