She did not pray during pregnancy out of ignorance

Firstly: 

The
pregnant woman is obliged to pray like any other woman who is in a state of
purity. Prayer is only waived for women in the case of menses or nifaas
(postpartum bleeding). Knowledge of that is widespread and readily
available, and ignorance of that is very strange, especially in Muslim
countries. Rather it is to be regarded as a shortcoming and negligence. 

What
every accountable individual must do is learn what he needs for his acts of
worship and interactions with others to be sound and valid. This is part of
the knowledge that it is obligatory to acquire, and it is not permissible to
delay it or be distracted from it. 

Therefore what your wife must do is repent to Allah, may He be exalted, for
her shortcomings and negligence in learning and asking people of knowledge.
But she does not have to make up these prayers, according to the more
correct scholarly opinion, regardless of whether she did not do them out of
ignorance or heedlessness, but she should strive to do a lot of acts of
worship and obedience, and a lot of naafil (supererogatory) prayers. 

Shaykh
al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 

Based
on that, if a person does not purify himself as he is obliged to because the
text did not reach him, such as if he eats camel meat and does not do wudoo’
after that, then the text reaches him and it becomes clear to him that
wudoo’ is required in this case, or he prays in the camel pens, then the
text reaches him and the ruling becomes clear to him, does he have to repeat
his past prayers? There are two scholarly opinions, both of which were
narrated from Ahmad. 

A
similar case is if he touches his penis then prays, then it becomes clear to
him that it is obligatory to do wudoo’ after touching one’s penis. 

The
correct view in all these cases is that he does not have to repeat the
prayers, because Allah pardons things done by mistake or because of
forgetting, and because He says (interpretation of the meaning): “And
We never punish until We have sent a Messenger (to give warning)”
[al-Isra’ 17:15].

If the
command of the Messenger (blessings and peace of Allah be upon him)
concerning a particular matter did not reach a person, then the ruling that
it is obligatory was not proven in his case. Hence in the case of ‘Umar and
‘Ammaar, when they became junub and ‘Umar did not pray whilst ‘Ammaar did
pray after rolling in the dust, the Prophet (blessings and peace of Allah be
upon him) did not instruct either of them to repeat the prayer. Similarly,
he did not instruct Abu Dharr to repeat the prayer when he used to become
junub and remained for several days without praying. And he did not instruct
the Companion who ate until he could clearly distinguish the white thread
from the black thread to make up the fast. And he did not instruct those who
prayed facing towards Jerusalem before news reached them that this ruling
(of facing Jerusalem when praying) had been abrogated to make up those
prayers. 

A
similar case is that of the woman who was suffering from istihaadah
(irregular, non-menstrual bleeding) and did not pray for a while because she
thought that prayer was not obligatory in her case. There are two scholarly
views as to whether she is obliged to make up the prayers. The first view is
that she is not required to do so – as was narrated from Maalik and others –
because when the woman who was suffering from istihaadah said to the Prophet
(blessings and peace of Allah be upon him): I bleed very heavily and that
has kept me from praying and fasting, he told her what she had to do in the
future, but he did not instruct her to make up prayers from the past. 

It is
proven to me through the mutawaatir reports that among women and men in the
desert and elsewhere there are those who reach the age of puberty and are
not aware that prayer is obligatory for them. In fact, if a woman is told to
pray, she says, Not until I am older and become an old woman! – thinking
that prayer is only obligatory for old women and the like. Among the
followers of the shaykhs (i.e., Sufis) there are many groups who do not know
that prayer is obligatory for them. In these cases they are not required to
make up the missed prayers, according to the correct opinion. End quote. 

Majmoo‘ al-Fataawa
(21/101, 102) 

Secondly: 

Nifaas
is the bleeding that occurs with childbirth, or two or three days before
childbirth, if it is accompanied by signs of impending childbirth, such as
contractions. 

It
says in Kashshaaf al-Qinaa‘ (1/219):

If she
sees blood three days or less before any part of the child emerges, and she
feels pain, then this is nifaas, like that which appears with childbirth.
End quote. 

Shaykh
Ibn ‘Uthaymeen (may Allah have mercy on him) was asked about a pregnant
woman: a few days before she gives birth she experiences bleeding and the
like; when does prayer become waived in her case? 

He
replied: 

The
scholars say that nifaas is the blood which appears at the time of
childbirth, accompanied by contractions. If the contractions come one or two
days before the birth, then this is nifaas. As for bleeding which occurs
without contractions, it is not nifaas, even if that occurs on the day that
she gives birth. When the bleeding of nifaas stops, some time after giving
birth, then when she becomes pure she must purify herself (ghusl) and start
to pray, and she should not wait for that time to be completed.

End
quote from al-Baab al-Maftooh (no. 31/8). 

If a
woman is about to give birth and she feels labour pains and starts to bleed,
then that is the blood of nifaas, and she should stop praying and fasting. 

But if
no blood appears, she should pray even if the cervix has started to dilate,
and she should carry on praying until she gives birth. 

And
Allah knows best.