We're on Earth, in an area whose weather is cold and dry most of the year, with a short summer and long winter (and brief transitional periods).
We have a mammal, around the size of humans, with a short weaning period.
The species doesn't hibernate but individuals are able to slow down and survive without a lot of calories or fluid intake. Infants, however, are completely dependent on their mothers and need a lot of calories and fluids in order to develop the thick skin/fur they need to survive winter. Their only food/fluid intake is breastmilk and they need it for as long as possible.
Mothers can produce milk briefly as summer transitions to winter, but then their milk dries up. There is little water available to drink (it's frozen) and their food supplies dwindle fast. Their bodies go into survival mode, which means they can not afford the calories/etc to produce milk.
In a land like this, babies conceived at the end of summer and born at the beginning of the next summer have the best chance of survival. Pregnant adults have plenty of food and fresh water available at the start of their pregnancies, which is enough to bring an embryo to the fetal stage before their winter slow down begins.
If the baby is born just after the start of summer, there will be enough food and fresh water for the mother to have the energy to give birth and to nurse her baby. The body grows and develops all summer and, come winter, is old enough to withstand a winter (or at least to have a fighting chance).
How does the fetus grow?
Gestation requires calories too; about as much as breastfeeding in some stages. The fetus may be protected from the cold all winter, it may not need fluids (what it needs is already there and the mother just needs enough for herself), but it still needs to develop and to grow larger.
Some of the caloric needs will come from the mother's fat stores, but this isn't enough. She needs most of those for her own survival.
Photosynthesis regulates the reproductive systems. Like with plants and many animals, the systems know the season based on the day length and quality of light. Only mid-summer (and a bit later) light can stimulate gonads. Males produce high quality sperm and females ovulate.
Photosynthesis regulates fetal growth. The energy generated by photosynthesis isn't enough for an adult to survive. It also isn't enough for a child, because their body's surface area is too small. But an adult can generate enough energy for a fetus. The fetus already is in place and has done most of its development (the second half or so of pregnancy is more gaining size than it is outright development, though there is some of that too). Photosynthesis gives the fetus the extra it needs to come to term.