Short Answer: This is Not Duress
This would not constitute duress or a ground for an annulment under U.S. law.
In general, economic considerations, or moral duties arising from love and affection, or a felt obligation to "do the right thing" following a pregnancy, or a need to bring other family members (or oneself) out of dire poverty, do not constitute duress that would constitute a ground for an annulment.
Duress would typically have to involve a threat of unlawful physical harm to the person getting married, or to a loved one of a person getting married.
Historically Duress Was Not A Ground For An Annulment
Historically, duress was not considered a ground for an annulment at all.
For example, historically, the duress of a "shotgun marriage" (literally, a threat to kill the man who got an unmarried woman pregnant if he didn't marry her), would not have been considered a ground for annulment, although it might be today.
Similarly, if a king married one of his daughters to the son of another king in exchange for not invading and ruining his country, the daughter could not annul that marriage, even if it was entered into because the two kings agreed that she would be killed if she did not agree and told her so, despite the fact that both she and her father were under duress at the time.
Annulments Are Rare And Are Rarely Sought For Duress
Annulment of a facially valid non-common law marriage is very rare.
For example, in 2017, there were 308 marriage annulment cases in Colorado and 5 civil union annulment cases, in a year in which Colorado had a population of 5,607,000 people. In the same year it has 25,290 divorce cases, 922 legal separation cases, 6,308 cases seeking an adjudication of parental rights and responsibilities involving unmarried couples, 1,957 paternity cases, 56 civil union dissolution cases, 5 civil union legal separation cases, and 6 cases seeking judicial permission for a minor to marry. There are approximately 41,500 marriages each year in Colorado, so the annulment rate is about 7 per thousand marriages in Colorado, which would be fairly typical nationally. There are about 12 annulments per thousand divorces in Colorado.
Duress would be grounds for an annulment in only a tiny share of those rare annulment cases that are filed.
In part, in the modern U.S., this is because the wide availability of unilateral no fault divorce, and the abolition of most marital immunities from civil and criminal liability, makes the use of duress to compel someone to marry a pretty futile exercise, that has a high potential for criminal law consequences for someone who attempts to use duress to force someone to get married.
Some typical grounds annulments other than duress would involve (in approximate order of frequency)
(1) the marriage was entered into under conditions that one or both of the parties felt was a joke or were temporarily too impaired to really understand because they were drunk or under the influence of drugs (legal or otherwise), or
(2) an individual who was married didn't have the capacity to appreciate fully what marriage means due to a developmental disability, or due to dementia, or
(3) the couple is legally prohibited from marrying (e.g. due to age, because they are a long lost brother and sister, or mother and son, or one or both of the parties was married to someone else at the time), or
(4) the marriage was entered into based upon a misunderstanding of one or both parties regarding the sex or gender of the other party (i.e. a woman who thought she was marrying a cisgender man but actually married a transgender man) or the identity of the party (the marriage ceremony was with the wrong twin, for example).
Duress would be a less common ground for an annulment than the first three reasons, but might be more common than the fourth reason set forth above.
Annulment And The Doctrine Of Laches
Also, annulment is equitable relief that can generally be barred by the doctrine of laches.
For example, if two people are married via a facially legal marriage license and aren't legally prohibited from marrying, did so while tripping on LSD and thinking that they were just joking, but then continue to live as husband and wife for eight years and have kids together, a court would probably deny them an annulment, even if an annulment would have been granted if the issue had been raised immediately, even in a state that is not a common law marriage state.
Similarly, a marriage entered into at age 13 that was legally recognized due to a lie on a marriage license application, would probably be annulled at age 18 or 20, but laches would probably prohibit an annulment of that marriage at age 40.
The "put everyone back where they started prior to marriage" premise of an annulment is easy to implement when people have been legally married for only a few days or months, especially if they have never actually shared their lives economically and haven't had kids together during the marriage, but is much harder to implement when they have lived together for a long time as husband and wife.
Footnote On Sham Marriages For Immigration Purposes
As an aside, an immigration agency determination that a marriage is a sham that does not afford the immigrant eligibility for citizenship is rarely, if ever, grounds for an annulment under other civil or criminal laws. For example, a sham marriage determination by immigration officials would not end eligibility to file a joint tax return, or end the ability to claim a spousal privilege to exclude a spouse's testimony in a court case, or change the presumption of paternity for children born during the marriage.
Canon Law Compared
Duress is grounds for an annulment for religious purposes under Roman Catholic canon law (Canon 1103), as are the circumstances described in the OP (Canon 1101, Section 2), which aren't considered duress under canon law either.
But, a church annulment would have no secular legal effect; it would only affect a person's status in regard to Roman Catholic prohibitions against divorce.
Why Seek An Annulment?
It is also unclear what you think that the legal ramifications of a divorce as opposed to an annulment would be in this case. The biggest would seem to be that Alice would lose any right to alimony or an equitable property division if this were an annulment rather than a divorce, so generally speaking, an annulment would be harmful to her legally.
Historically, annulment was, in part, an extraordinary remedy for cases where divorce was simply not available at all, at least without special legislation authorizing the divorce of a particular man and a particular woman, and in part, a natural outgrowth of the process of a court trying to determine as a factual finding, whether or not two people had gotten married, or had not gotten married. But, in an age where no fault divorce is widely available on an essentially unilateral basis, the need for annulments has largely evaporated.
