So you want to have babies, do you?? Well, there's a few things you should know first...
To begin with, pony breedings cost 20k each (10k per owner if there are two owners), and pretty much happen by magic! This means that the gender of the partners wanting children doesn't matter. It also means species doesn't matter (so far as one species being larger or smaller or anything like that). The only exception to this is that Dream & Nightmare Legends cannot breed with each other. How is this possible, you ask??
Well... you see. In various areas of the My Little Pony world, there are small areas where the magic that formed the world is particularly strong. No one can ever really predict where they are... but if two ponies happen to be close to one of these areas and they like each other... well, then. They might just suddenly find later on that the stork has dropped off some little ones!
The number of babies possible in all normal breedings is two to four. There are some special circumstances, though, so keep reading!

:: General Rules ::
1 ~ Since ponies have to like one another for the magic of breeding to work, please let's not have anything like rape or forced mating, etc. :C That's just not any fun. This doesn't mean that your ponies can't be mean/trick others/be rough if you so wish, but that's not exactly the same thing. If two ponies LIKE well ... pain and such... then that's fine, it's not one forcing another to do something they REALLY don't want to do. This also goes for One Night Stands. Those are totally still allowed, they just can't be forced.
2 ~ From this point forward, all ponies have a breeding limit of three for any breedings that are NOT RPed. This means that each pony may only breed three times UNLESS they RP and send in logs with their breeding request. If the breedings are RP breedings, they are pretty much UNLIMITED. Any ponies who have had babies before Feb. 2008 get to "start over". Meaning that no matter how many breedings they had before this point, they now have three more non-RP breedings and then they may not breed again unless the breedings are RP breedings.
3 ~ Send your trade of 20k to the appropriate colorist as soon as your request is approved! Breedings will not be started without payment!
4~ Your pony MUST be named and certed before they can breed.
5 ~ Any pair of ponies that has babies from a previous breeding may not breed until those babies are at least teens. And it's preferred that previous babies be adults before ponies breed again.
6~ If you have babies from a previous breeding those must be named and their information posted in a certing thread/sent to colorist before you may get another breeding. This is only once you HAVE the uncerted babies. Any point in time slots are open and the breeding you've ordered is still waiting to be rolled/colored/whatever you may order another. You may also still (if slots permit) order more than one breeding at a time.
:: On Babies ::
1 ~ In the case of single-owner breedings... One baby must ALWAYS be given away. The only exception to this is if there are only two babies. Then the single owner may choose to keep both, but it is preferred that they still give one away.
2 ~ For breedings in which two owners are involved, each owner may keep up to two babies UNLESS the breeding is an RP breeding, in which it's possible for owners to keep three (if five or six are rolled).
3 ~ If there is an uneven number of babies in ANY breeding, owners are encouraged to give away the "extra" baby. This isn't necessarily a rule, but it's suggested just to keep everything fair. If owners wish to work out between themselves who gets the "extra" baby, they are still allowed to do so.
:: RP Rules ::
1 ~ If you feel the need to actually RP mating, DO NOT DO IT ON GAIA. This violates the TOS since it would most likely be over PG-13. If you like, you may RP up to that point and then "fade away" and do the rest somewhere else.

:: Special! ::
So, I said up there that there are "normally" two to four babies, right..? Well, there's a way to receive up to six babies, if you're willing to do a bit of work! How, you ask...?
The answer is RP! The more you RP, the more possibilities there are for more babies. Here's how it works...
When you submit a breeding form, if you include five documented and finished RPs, you will get one extra baby over whatever is rolled by the dice. This means you could get up to five! Documented means that you need to quote thread RP in a journal, or RP in the guild, or something like that ... just be able to post a link to the RP and be sure it's finished!
Also, if you submit a total of ten documented and finished RPs, you will get two extra babies over whatever the roll is. Which means you could get up to six! Exciting, huh?
Example 1 ~ Pony A and Pony B are breeding and have submitted 5 RP logs. Dice is rolled... and the dice say they get 3 babies! So ... since there are 5 logs, they will actually get 4 babies.
Example 2 ~ Pony C and Pony D are breeding and have submitted 10 RP logs! Dice is rolled... and the dice actually say 4 babies! But since there are 10 RP logs, 2 extras are added...which means they get 6!
For long RP logs, you could always split it up at logical places when you submit it in order to say that it's 2-3-5 "finished" RPs. A finished RP needs to be at least 5-6 posts per person.
For OOC RP, if you want that to count, it needs to be at least 10-15 posts long. And preferably, each post should be at least a sentence or two, no matter if you do it OOC or IC
Here are a couple of examples of logging~~
Mandas Kitsu RP logs
Mandas SoA RP logs