She committed zina before her wedding and got pregnant, then had a miscarriage. Is her marriage valid?

Firstly: 

If this immoral action came
after the marriage contract was done, even if it was before the wedding
party, then this marriage of yours is valid but you have to repent from this
sin that you fell into. 

Ibn Qudaamah (may Allah
have mercy on him) said: 

If the wife of a man
commits zina, or her husband commits zina, the marriage contract is not
rendered invalid, whether that happens before or after consummation,
according to the opinion of most of the scholars.

End quote from al-Mughni,
9/565 

Secondly: 

If this zina took place
before the marriage contract was done, the marriage contract is not valid
unless it was done after establishing that there was no pregnancy by waiting
for one menstrual cycle, according to the more correct opinion. 

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

Marriage to a zaaniyah (a
woman who has committed fornication) is haraam until she repents, whether
the one who committed zina with her is the bridegroom himself or someone
else. This is the correct opinion beyond a doubt, and it is the opinion of a
number of the earlier and later scholars, including Ahmad ibn Hanbal and
others. Many of the earlier and later scholars were of the view that it is
permissible, and this is the view of the three, but Maalik stipulated that
it should be established that there is no pregnancy by waiting for one
menstrual cycle, and Abu Haneefah regarded it as permissible to do the
marriage contract before waiting for one menstrual cycle if she is pregnant,
however if she is pregnant it is not permissible to have intercourse with
her until she gives birth. Ash-Shaafa‘i regarded it as permissible to do the
marriage contract and to have intercourse in all cases, because the water
(semen) of the zaani has no value in sharee‘ah and the ruling is that no
child is to be attributed to him. This is his justification for his opinion.
Abu Haneefah differentiated between the one who is pregnant and the one who
is not, because if (the husband) has intercourse with the one who is
pregnant, he will be attributing to himself a child who definitely is not
his, which is different from the case of one who is not pregnant. Maalik and
Ahmad stipulated that it should be established that there is no pregnancy
(by waiting for one menstrual cycle), which is the correct view. But Maalik
and Ahmad, according to another report, stipulated that it should be
established that there is no pregnancy by waiting for one menstrual cycle.
The other report from Ahmad is that which is followed by many of his
companions such as al-Qaadi Abu Ya‘la and his followers, which is that it is
essential to wait for three menstrual cycles. However the correct view is
that all that is required is to establish that there is no pregnancy (by
waiting for one menstrual cycle). End quote. 

Majmoo‘ al-Fataawa,
32/110 

Shaykh Ibn ‘Uthaymeen (may
Allah have mercy on him) said in ash-Sharh al-Mumti‘ (13/382): Rather
the opinion narrated from Abu Bakr and a number of the Sahaabah (may Allah
be pleased with them) is that there is no ‘iddah at all for the woman who
committed zina, and there is no requirement to establish that there is no
pregnancy, especially if she has a husband, because of the words of the
Messenger (blessings and peace of Allah be upon him): “The infant is to be
attributed to the husband of the woman (who gave birth to him).” Rather if a
man knows that his wife has committed zina – Allah forbid – and has
repented, he should have intercourse with her immediately, so that no doubt
will remain in his heart in the future as to whether she became pregnant as
a result of zina or not. If he has intercourse with her immediately, the
child will be assumed to be from the husband and not from the zaani
(adulterer). 

But if the woman who
committed zina (fornication) did not have a husband, it is essential to
establish that there is no pregnancy by waiting for one menstrual cycle,
according to the correct opinion. End quote. 

Thirdly: 

Because this issue is the
subject of a considerable difference of opinion among the scholars, and
because annulling the marriage and admitting zina after this length of time
will lead to a great deal of trouble and disclosing that which Allah had
concealed for you, and will expose you to turmoil, we think – and Allah
knows best – that in this case you do not have to tell your husband about
what happened in order to renew the marriage contract. In the other
scholarly opinion mentioned above, according to which some of the scholars
are of the opinion that it is not essential to establish that there is no
pregnancy, there is some leeway. 

This, as we have said,
applies if the zina took place before the marriage contract was done and
before it was established that there was no pregnancy. 

As Allah has concealed you,
you must also conceal yourself and do not breach the concealment of Allah.
al-Bukhaari (6069) and Muslim (2990) narrated that Abu Hurayrah (may Allah
be pleased with him) said: I heard the Messenger of Allah (blessings and
peace of Allah be upon him) say: “All of my ummah will be fine except those
who commit sin openly, and it is part of committing sin openly for a man to
do something at night, then in the morning when his Lord has concealed him
he says: O So and so, I did such and such last night, when his Lord had
concealed him all night, but in the morning he discloses that which Allaah
had concealed for him.” 

And Muslim (2590) narrated
from Abu Hurayrah (may Allah be pleased with him) that the Prophet
(blessings and peace of Allah be upon him) said: “Allaah does not conceal a
person in this world but Allaah will conceal him on the Day of
Resurrection.” 

Al-Bayhaqi (18056) narrated
from Ibn ‘Umar that the Messenger of Allah (blessings and peace of Allah be
upon him) said, after the stoning of al-Aslami: “Avoid this filth that Allah
has forbidden, and whoever falls into it, let him conceal himself with the
concealment of Allah, may He be glorified and exalted.”

Classed as saheeh by al-Albaani
in as-Saheehah (663). 

Fourthly: 

Your son is a legitimate
son of your husband, and there is no problem concerning him, in sha Allah. 

With regard to the foetus
whom you aborted, if that happened before the soul was breathed into him –
i.e., before four months – then there is no expiation or diyah required for
him. However you are required to repent, feel regret and pray for
forgiveness. 

If that happened after four
months, then you have to pay the diyah and offer expiation.

The diyah is to free a male
or female slave; if that is not possible then you must pay the equivalent,
which is five camels.

With regard to the
expiation, it is to free a slave. If that is not possible, then you must
fast for two consecutive months.

See also the answer to
question no. 106448

And Allah knows best.