No current theory can explain the changes, it seems more the conclusion of a science fiction novel than real world physics. The fact is that on D-day, such an appropriate name, the first cases of change had been reported in Australia and every 20 days it shifted time zone, until the changes reached the whole globe.
Laura Silverman, was the first recorded case, although her name had been mostly forgotten, drowned in the literal billions of similar cases. She was just regular office worker in a secretarial job, when one day she called sick and went to a doctor for an appointment, something completly outside Ms. Silverman perfect attendance. The hospital at first had to check to see if there wasn't any fraud against the health system. Laura Silverman was registered as a single female in every single oficial document, no sign of major cirurgy in her medical history nor any rectification in her documments, however, the patient clearly had male genitals instead of the female anatomy.
The local authorities never pursued the case any further, for just after Laura Silverman's case, a flood of new reports came in. Not only in Australia, but in the whole time zone, similar cases popped up everywhere. In 20 days after the first confirmed case every single woman, despite age, etnicity or social-economic group had been inflicted with the same condition.
The condition spreaded following the time zones in a perfect timetable, marching from east to west, no prevention measures took effect against the changes, it didn't seem to have a viral nature. One or more cases would be confirmed in a place and short after every single woman in the time zone would have her genitalia changed into a male version, although in terms of size they would be in average 35% larger than a man's version of the same equipment. After 20 days there would be no woman with the traditional female organs left.
At first pregnant women seemed to be immune to the condition, which lead a lot of pregnancies in the hopes of keeping the condition form spreading. Many women also just wanted to be mothers before they lost capacity forever. Unfortunately, a pregnancy would only delay the change, short after giving birth the woman would change as well in less than 24h.
After half a year a full crisis had been instaured worldwide. With the exception of a few woman still pregnant, there were just no more women with wombs. How would humanity keep on going?