Well I guess the advatages of this are :
1-life is becoming expensive and more than two kids will cost the parents more money to spend to raise them up.
2-if parents are both working for 8 hrs a day they need more time to sit with more than two kids ,talk to them feel them undestand then solve thier problems, teach them, go outside with them
3-thier education money should saved and provided.and if the family has more than two they have to spend more money.
4-nowadays finding a high salary job is not easy to provide life things for the kids so the main issue here is money and time
5-for mothers more effort should be spent to born kids,feed them spend nights with them, teach them many things like wearing cloths, way of talking,playing ,walking, handling things, treat them when they are sick........ect
so being a mother is not easy
Well it just my openion from my family experience