She got pregnant before she started to make up missed fasts, and she cannot fast

My wife owed some daysthat she did not fast last Ramadaan because of
her menses, but before she formed the intentionto fast those days
beforethe next Ramadaan, she got pregnant, and the doctor told her
that she should not fast at all during her pregnancy and possibly
during the breastfeeding period too, because of her general weakness
and fearing for the foetus. Hence she will not be able to fast these
days. What should she do about these days? What should she do if she
cannot make up the daysof the next Ramadaan before the following
Ramadaan comes?.
Praise be to Allaah.
Allaah has given the one who missed some fasts because of a legitimate
excuse until the following Ramadaan to make up those fasts, but the
Muslim should not use this as an excuse to delay making them up,
because he may be facedwith some need or changes that make it toohard
for him to make them up or prevent him from doing so, especiallywomen
who may become pregnant, or be faced with menstruationand nifaas
(postpartum bleeding).
Whoever delays making up the fast with no legitimate excuse until
there is very little time left, then Sha'ban ends and he has not made
up the fasts he owes, is sinning, but if he has an excuse then there
is no sin on him. In either casehe has to make up those fasts after
the second Ramadaan. Some scholars stated that he also has to feed one
poor person for each day. If he is able to do that then that is on the
safe side, otherwise making up the missed fasts is sufficient.
See also questions no. 26865 and 21710 .
Shaykh Muhammad Saalih al-'Uthaymeen (may Allaah have mercy on him)
was asked: Whatis the ruling on one whodelays making up missedfasts
until the next Ramadaan comes?
He replied:
Delaying making up missed Ramadaan fasts until the next Ramadaan comes
is not permissible according to the most well known scholarly opinion,
because 'Aa'ishah (may Allaah be pleased with her) said: "Iwould owe
Ramadaan fasts and I would not be able to make them up except in
Sha'baan." Thisindicates that there is noconcession after the second
Ramadaan. If a person does that with no excuse, then he is sinning,
and he must hasten to make up thosefasts after the second Ramadaan.
The scholars differed as to whether he must also feed the poor or not.
The correct view is that he does not have to feed the poor as well,
because Allaah says(interpretation of the meaning):
"but if any of you is ill oron a journey, the same number (should be
made up) from other days"
[al-Baqarah 2:184]
Thus Allaah did not enjoin anything on people in this situation apart
from making up the missed fasts.
Majmoo' Fataawa al-Shaykh Ibn 'Uthaymeen, 19, question no. 357.
The Shaykh was also asked:
A woman did not fast some days of Ramadaan last year, then she made
them up at the end of Sha'baan. Then her period came and lasted until
Ramadaan began this year, and she still had one day to make up.What
should she do?
He replied:
She has to make up that day that she was not able to make up before
Ramadaan began this year. When this Ramadaan ends, she has to make up
what she missed from the Ramadaan of the previous year.
Majmoo' Fataawa al-Shaykh Ibn 'Uthaymeen, 19, question no. 358.
And he was asked:
A woman did not fast during Ramadaan because of nifaas (postpartum
bleeding), and she was not able to make up the fasts beforethe next
Ramadaan came because of breastfeeding. What should she do?
He replied:
This woman should fast and make up the days that she missed, even if
that comes after the second Ramadaan, because she was unable to make
up the fasts between the first and second Ramadaans for legitimate
reasons. But ifit is not difficult for her to make up the fasts during
the winter, even if she does it day after day, then she has to do
that, even if she is breastfeeding. She should strive to do whatever
she can to make up the missed Ramadaan fasts before the next Ramadaan
comes. If she cannot manage to do that then there is no sin on her if
she delays it until the next Ramadaan.
Majmoo' Fataawa al-Shaykh Ibn 'Uthaymeen, 19/ question no. 360.
To sum up: These days remain a debt which your wife owes, and which
she must make upwhenever she can.
And Allaah knows best.