Maternal or Maternity refers to when a woman is pregnant or becomes a mother
Pregnancy is when you are carrying or due to give birth to the baby so if you have a photo taken when pregnant, for which a lot of women do have photoshoots, this is called a 'pregnancy photo'
Maternity can refer to motherhood in general, but most commonly is used to refer to the time period in which a woman is pregnant or has very recently given birth. Photos is the shortened way of saying photographs.
Pregnancy pictures refers to photographs that are taken to preserve the memory of the time period shortly before childbirth.
Baby bump pics is the most slang way to express maternity photos. "Baby bump" is how people refer to a woman's mid-section when it has expanded because of being pregnant. Pics is the slang term for photographs.
When a pregnant woman gets her pictures taken, it's usually called a "maternity photo shoot" or "maternity photos".
You may use the above terms in a sentence in the following ways:
-My pregnant sister has a maternity photo shoot planned and she wants us all to be there.
-My sister's maternity photos look great.
妊娠した女性が写真を撮ってもらうことは、一般的に "maternity photo shoot" または "maternity photos" と呼ばれます。
上記の表現は次のように使えます。
-My pregnant sister has a maternity photo shoot planned and she wants us all to be there.
(妊娠中の姉がマタニティフォトを撮ってもらうことになっていて、私たちみんなにも同席して欲しいと言っています)
-My sister's maternity photos look great.
(私の姉のマタニティフォトは素晴らしいです)
If a pregnant women is getting their photo taken, for example in an arranged photoshoot to celebrate their upcoming child, you could refer to this in English as one of the following: "Maternity photos", "Pregnancy photos" or "Baby bump pictures". If you used any of these three examples, the person you are speaking to in English would understand.
"Maternity" is used as an adjective for women preparing to give birth or for a woman who has recently given birth. We use it to describe clothes for pregnancy (maternity clothes) or time off work (maternity leave) after having the baby.
There is not much difference in terms of understanding between "maternity photos" and "pregnancy photos". You can use the terms interchangeably.