Ruling On Obeying Parents in Refraining from Having Children
Question

The wife wants to have a third baby, and her husband says: Whatever you wish, and she feels that he wants to have another child too, but the problem is that her mother rejects this idea and argues with her, and may cut off ties with her because of that. What do you advise her to do – should she pursue her wishes and have another child, or should she refrain and obey her mother?
Praise be to Allah.Muslims are urged to have a lot of children 
The teachings of Islam urge Muslims to have a lot of children and encourage them to do so , because having many children is a source of pride and strength for the ummah, and is something of which the Prophet (blessings and peace of Allah be upon him) will feel proud on the Day of Resurrection. 
Abu Dawud (2050) narrated that Ma‘qil ibn Yasar (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) said: “Marry women who are loving and fertile , for I will boast of your large numbers before the nations [on the Day of Resurrection].” (Classed as sahih by al-Albani in Irwa al-Ghalil, 1784)
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said:
“What the Muslims should do is have as many children as they can, because this is what the Prophet (blessings and peace of Allah be upon him) advised them to do, when he said: “Marry women who are loving and fertile, for I will boast of your large numbers.” Moreover, having a lot of children will increase the numbers of the ummah, and the large numbers of the ummah are a source of pride for them, as Allah, may He be exalted, said, reminding the Children of Israel about that (interpretation of the meaning):
{And We reinforced you with wealth and sons and made you more numerous in manpower.} [al-Isra’ 17:6] 
And Shu‘ayb said to his people:
{And remember when you were few and He increased you.} [al-A'raf 7:86]
No one can deny that the large numbers of the ummah are a source of pride and strength for it, in contrast to what those who think negatively imagine, who think that the large numbers of the ummah are a cause of its poverty and hunger.” (Fatawa Islamiyyah, 3/190)
Obeying parents in refraining from having children: Required?
The child does not have to obey his parents in refraining from having children, for two reasons:
1.    It is a command that is contrary to the command of the Prophet (blessings and peace of Allah be upon him).
2.    Having children is a right that is shared by both spouses, and no one else has the right to interfere in that.
Nevertheless, the wife must show kindness to her mother and speak gently to her.
For more, please see these answers: 1176 , 214117 , 12199 ,and 9155 .
And Allah knows best