Unfortunately for us, the people who knew the most about preventing and ending pregnancies in the Middle Ages were women, especially women who weren’t in a position to be writing things we would have now.  But even so, we do know that people who wanted to prevent pregnancy had several options open to them, some more effective than others.  A lot of this information actually comes from some of the very authors who disapproved of the use of contraception (though largely as an afterthought or side issue to more important topics), some of them seemingly following the saying “if not chastely, then at least cautiously.”  Some methods were more effective than others.
 
There are a couple different plants that were commonly used to prevent pregnancy or stop it in its first few days.*  Silphium, so famous in earlier centuries, was extinct other plants were known to work as well.  Pennyroyal seems to have been the most widespread, but Queen Anne’s lace (also known as wild carrot) and rue seem to have been widely used as well.  Some plants, like asafetida (a relative of silphium) have been proven to be somewhat effective, but not totally so. Pomegranate was also believed to act as a contraceptive, but that has since been disproven.  The problem with most of these was how toxic they were.  An overdose could easily result in far more serious health problem, or even death.
 
Not that these things were always used for contraception.  Many of them were known to bring on menstruation, which could be done to prevent a pregnancy, but it could also be used to make sure of fertility (if, for example, a woman hadn’t been having periods in a while) or to easily remove an already dead fetus from the body.
 
Another method of contraception was to have sex in such a way that wouldn’t get someone pregnant.  Coitus interruptus is known to have been a common method, but pennitentials and other texts also mention oral and anal sex, among other ways.  Some people tried wearing amulets and charms or jumping up and down after sex.  More effective than either of these things was the insertion of a sponge or some other barrier into the vagina.  Regardless of the Church’s (rather formulaic) disapproval, medieval people knew and practiced a wide variety of contraceptive methods, some of which were more effective than others.
 
 
*Most medieval people who had opinions on the subject that we know about seem to have considered the fetus a person either after 40 days or once its mother felt it move.

Sources/Further Reading:
Brundage, James A. Law, Sex and Christian Society in Medieval Europe. Chicago: University of Chicago Press, 1987.
Birth Control and Abortion in the Middle Ages - Medievalists.net
Contraception (Sample article from Women and Gender in Medieval Europe: An Encyclopedia) - Routledge

Note: My most sincere apologies for the lateness of this post.

When a peasant woman married, there were either one or two notable expenses besides the wedding itself: the dowry and the merchet.  This second item was a fee paid to the lord on the marriage of an unfree peasant woman, essentially intended to counter her loss as a worker and some land as dowry.  How much the merchet actually was depended on the lord, his bailiff, the woman’s own property, and the size of her dowry.  Non-payment, no matter the size, could result in the seizure of all of her family’s property.
 
The thing is, merchet applied only to two groups of peasants: unfree peasants (i.e. serfs) and peasants who held land that was designated unfree but were themselves free.  In other words, the obligations of work and taxes that were required of serfs could also be tied to the land itself instead of or as well as to the people who worked it.*  So technically the lord or his bailiff could demand merchet from a free peasant woman, provided her family held unfree land.  How often that happened though is unknown.
 
It’s also worth noting, though, that not all unfree peasants paid the merchet either.  Before the 13th century (and perhaps even after in more out-of-the-way places) there was the question of clandestine marriage.  Some people might be declared married and required to pay the merchet, but others might simply live together as married without anything official.  Also, as time went by some families managed to conceal their serf status and get out a lot of their obligations.**
 
Then there was the question of dowry and who the woman married.  The amount owed for merchet often depended on the size of the dowry.  A woman with a dowry small enough might not have to pay anything.  If she married someone under the lord’s jurisdiction, the merchet would be smaller.  If she married farther afield, though, the payment was correspondingly larger.
 
A lot depended on circumstance and location.  The amount owed varied depending on who the woman married, where she lived, and what her family owned.  Some families managed to hide the fact that they owed merchet entirely, either by obscuring their serf status or obscuring whether or not there was actually a marriage.


*There are a number of ways a free peasant could come to hold unfree land.  To name a few, they could inherit it from a relative who was a serf, or even from a free relative who already held unfree land.  Or they could buy it from another free peasant.
**In some cases it became a secret they were willing to kill to protect.

Sources/Further Reading:
Payment of Merchet, Pope Gregory the Great - Internet History Sourcebooks Project
Searle, Eleanor. "Seigneruial Control of Women's Marriage: The Antecedents and Function of Merchet in England." Past and Present82 (1979): 3-43.
Gies, Frances and Joseph Gies. Marriage and Family i the Middle Ages. New York: Harper & Row, 1987.

Illegitimate children in Ancient Rome were the offspring of any two parents who weren’t bound by Roman marriage.  And this was specifically Roman marriage, so the children of concubines, citizen/non-citizen pairs, and other couples living in de-facto marriages (see here and here) were all illegitimate under Roman law, regardless of whether their parents considered themselves wed.  Unlike in later periods, there was no particular social stigma against illegitimacy, but they received less from public assistance programs and were subject to different laws regarding patria potestas, citizenship, and inheritance, some of which placed them at a disadvantage.  Most notably, legitimate children belonged to their father and his family, while illegitimate ones belonged to the mother and her family.*
 
A couple things follow from this.  First, where legitimate children were under the patria potestas of their father, illegitimate children had no famila and were in no one’s power unless their father or some other man legally adopted them.  Second, they inherited their status and citizenship from their mother.  So if she was free, her children were freeborn.  If she was a slave, so were they.  If the mother was a Roman citizen, so too were her children.  So, for example, if a free citizen woman had children with a non-citizen man,** they would be free citizens, regardless of his free or slave status.***  Third, since they were legally their mother’s offspring but not their father’s, under the Leges Iuliae they counted towards the number of children she needed to be free from guardianship, but did not contribute to the benefits a man could get from having more children.
 
Since they weren’t part of their father’s family, illegitimate children couldn’t inherit from him if he died without making a will.  And until the second century CE, the same went for the inheritance of a woman’s property (except her dowry) by any of her children, legitimate or illegitimate.  Unless she made a will, any of a woman’s children had as much right to her property as her parents and siblings, sometimes less.  It was only in the second century CE that the law changed, giving children priority in the inheritance of their mother’s property if she died intestate.


*A married woman’s husband had the right to choose whether to raise a child or expose it.  Children he raised were usually considered to be his own.
**Since under Roman law marriage was only between two citizens, the children of any relationship between a citizen and a non-citizen would always be illegitimate.
***Marriage-like relationships between free women and slave men were common enough to seriously concern certain moralists, especially 2nd-4th centuries.  These de-facto marriages, along with those between two slaves, were known as contubernium.

Sources/Further Reading:
Gardner, Jane F. "Legal Stumbling-Blocks for Lower-Class Families in Rome." In The Roman Family in Italy: Status, Sentiment, Space, edited by Beryl Rawson and Paul Weaver, 35-54. Oxford: Oxford University Press, 1999.
Rawson, Beryl. "Adult-Child Relationships in Roman Society." In Marriage, Divorce and Children in Ancient Rome, edited by Beryl Rawson, 7-30. Oxford: Oxford University Press, 1991.
Pomeroy, Sarah B.  Goddesses, Whores, Wives, and Slaves. New York: Schocken, 1995.

Since divorce, widow(er)hood, and remarriage were so common in Rome, it should come as no surprise that what we would now call “blended families” were very common and, quite frankly, ordinary.  This isn’t to say that nobody gave any thought to it or that there wasn’t concern and anxiety over the potential preference for blood relations over step-relations (e.g. the fear that a woman would give her blood children preferential treatment over her stepchildren was not uncommon), but it also created networks between families, which could potentially be used to political advantage.
 
When a marriage ended, the children stayed with and were legally part of their father’s family.  So a woman who remarried generally would not raise her own biological children.  When Pompey divorced his third wife Mucia Tertia, her children (then in their early teens) stayed with him.  She herself remarried the brother of Pompey’s second wife, Aemeluis Scaurus and had children by him as well.  Though now attached to a different family, we might wonder how much connection she still had to her children by Pompey.
 
Many remarriages seem to have happened only once the children of the previous union were at least in their early teens.  When Julia Caesaris became Pompey’s fourth wife, his children by Mucia gained a stepmother very close to their own age.  Marcia’s second husband had a daughter, Hortensia, old enough to already have been widowed, by the time she married him.
 
There were also, however, several cases of people raising quite young stepchildren.  Marcia’s own father, Lucius Marcius Phillipus, only remarried to Atia Balba Casonia after Marcia herself had married Cato the Younger, though in this case he gained two young stepchildren: Octavia Minor and Gaius Octavius.  Octavia herself wound up raising quite the large brood of children including her biological children by Marcellus and Antony and her stepchildren through Antony’s unions with Fulvia and Cleopatra VII.

Sources/Further Reading:
Bradley, K. R. "Remarriage and the Structure of the Upper-Class Roman Family." In Marriage, Divorce, and Children in Ancient Rome, edited by Beryl Rawson, 79-98. Oxford: Oxford University Press, 1991.
Corbier, Mireille. "Divorce and Adoption as Roman Familial Strategies. In Marriage, Divorce, and Children in Ancient Rome, edited by Beryl Rawson, 47-78. Oxford: Oxford University Press, 1991.
Pomeroy, Sarah. Goddesses, Whores, Wives, and Slaves: Women in Classical Antiquity. New York: Pantheon Books, 1995.

Despite the idealization of the univira, a woman only once-married, in practice things were generally rather different.  Remarriage of both women and men in Rome during the Late Republic and Early Empire seems to have been relatively common, especially in the upper classes.*  And since a marriage was expected to produce children, it should hardly be surprising that what we call “blended families” were normal enough to not be commented on much.

The number of marriages that ended with the death of a spouse in battle, childbirth, or disease was not small, while, among the upper classes especially, divorce was not uncommon, especially for political reasons, but also for moral ones.  For men there was the simple expectation that marrying again would provide political advantages, largely in terms of gaining and holding political allies.  Pompey the Great’s third marriage, to Julia Caesaris, bound him to her father, Julius Caesar.  Her death seriously weakened their alliance, the breaking of which led to civil war.

For women, the ideal of the univira competed with economic and political necessity sometimes combined with pressure from relatives to remarry.  Unless she already controlled her inheritance, as Cornelia Africana did, it would be difficult for her to survive on her own.  If she was still in her father’s power, he might well urge her to remarry or simply arrange another politically beneficial marriage for her.  By the 1st century BCE, the Augustan marriage laws made remarriage an even more attractive option by demanding higher taxes from the unmarried than from the married.

Some divorces, namely those for adultery, made it less likely that a woman would find a new spouse afterwards.  Others happened specifically so that one partner could marry someone else.  Livia Drusilla’s divorce from her first husband, for example, was immediately followed by her marriage to Octavian in 38 BCE.  Aside from those few women who took on the ideal of the univira in their widowhood or could not find a second husband, remarriage was encouraged.


*For whom, as usual, we have the most evidence.

Sources/Further Reading:
Bradley, K. R. "Remarriage and the Structure of the Upper-Class Roman Family." In Marriage, Divorce, and Children in Ancient Rome, edited by Beryl Rawson, 79-98. Oxford: Oxford University Press, 1991.
Corbier, Mireille. "Divorce and Adoption as Roman Familial Strategies. In Marriage, Divorce, and Children in Ancient Rome, edited by Beryl Rawson, 47-78. Oxford: Oxford University Press, 1991.
Pomeroy, Sarah. Goddesses, Whores, Wives, and Slaves: Women in Classical Antiquity. New York: Pantheon Books, 1995.

The legitimacy of children was, in Ancient Rome, an issue of concern, though it had perhaps less stigma associated with it than it would in later eras.  By definition, any child born outside of a valid, Roman marriage between citizens was considered illegitimate.  In other words, no matter how valid the relationships of their parents, the children of concubines, contubernales, and Roman citizens who wed non-citizens were all denied the legitimate status.

Much of what we know about illegitimacy concerns the wealthier classes.  As far as we can tell, it was not that much of an issue during the Republic.  Recorded examples of illegitimate children are few, and it’s likely that most such children were aborted, exposed, or simply raised as though they were the legitimate children of the mother’s husband.  By the time of the Empire illegitimacy was a bit more of a concern.  Augustus and other emperors created laws criminalizing adultery and regulating inheritance by illegitimate children.  How strongly these were enforced though, depended on the emperor.

Originally, illegitimate children weren’t entitled to any inheritance unless they were named in a will.  Eventually, they were allowed to inherit from their mothers even without a will, where before property had gone first to legitimate children, then to her male relatives (brothers, fathers, uncles, cousins).  At this point the mother of an illegitimate child was also allowed to inherit from her child if they died intestate.*

Among the lower classes, one’s status as freeborn, freed, or unfree was far more important and relevant than legitimacy.  In the case of a child born outside of a valid Roman marriage, the child followed the status of the mother.  The child of a slave was born a slave.  The child of a free woman** was freeborn.  This would cause some concern with the rising numbers of free women living as the contubernales of highly influential slave men.  The child’s illegitimacy was only really important here in that it meant they inherited their mother’s free status rather than being unfree like their father.

*Without making a will.

**Or a woman freed and subsequently enslaved again during her pregnancy.  Legally, at least.  How that worked out in practice was most likely another matter entirely.

Sources/Further Reading:
Edwards, Catharine. The Politics of Immorality in Ancient Rome. Cambridge: Cambridge University Press: 1993.
Rawson, Beryl. The Family in Ancient Rome: New Perspectives. Ithaca, NY: Cornell University Press, 1986.
McGinn, Thomas A.J. Prostitution, Sexuality, and the Law in Ancient Rome. New York: Oxford University Press, 1998.

Many Hellenistic queens are known for their plotting and scheming on behalf of their sons.  It was often at least partially through a mother’s efforts that her son took his father’s place on the throne.  Hellenistic queens were often close to their sons, less so to their husbands.  A major reason for all of this was the fact that many Hellenistic kings practiced polygamy and married multiple wives.

This was probably a holdover from Macedonian practice and seems to have been a relatively fluid system.  Most kings didn’t designate a primary wife and neither wife nor son’s prominence was certain.  A favored wife could find her power given to another.  A son who was named heir presumptive could see that position taken from him by his father and bestowed on another.  Half-brothers scrambled to prove each other illegitimate.  Infighting was semi-constant and power never secure.

Why would a king risk that sort of instability?  For several reasons.  Having multiple wives allowed for the possibility of multiple marriage alliances with both local families and foreign powers.  It also meant that if one wife didn’t produce any male heirs, another might.  The potential for infighting was preferable to the lack of an heir.

What about the women themselves?  Marriage alliances could benefit a wife almost as much as they did her husband, especially if her family was physically nearby.  If she had a son, she could rise to become the king’s most important wife and then maybe even the next king’s mother, both positions of significant power.  Even if a wife had no sons, she could ally herself with one who did and obtain power and prestige that way.

Some examples are pretty clear-cut.  Phillip II had seven wives, the most famous of whom, Olympias, was the mother of Alexander the Great and a fierce political player herself.  Ptolemy VIII married both his sister Cleopatra II and his niece Cleopatra III.  The marriages of Antiochos II first to Laodike I, then to Berenike, follow more in terms of serial monogamy, but his return to Laodike might share continuity with his predecessors’ practice.

Sources/Further Reading:
Pomeroy, Sarah. Goddesses, Whores, Wives, and Slaves: Women in Classical Antiquity. New York: Pantheon Books, 1995.
Shipley, Graham. The Greek World after Alexander 323-30 B.C. New York: Routledge, 2000.
Carney, Elizabeth Donnelly. Women and Monarchy in Macedonia. University of Oklahoma Press, 2000. [Can be found on Google Books here.]
Review of Ogden, Polygamy, Prostitutes, and Death: the Hellenistic Dynasties - Bryn Mawr Classical Review

From a legal perspective, getting a divorce was relatively easy for an Athenian citizen.  Marriage and divorce were largely regarded as a private, familial affair.  These things were contracted between families with little to no involvement by authorities.  The state only stepped when conflict arose or public verification of a woman’s statement was needed.  Socially though, a divorce could be quite difficult to get and as a result seems to have remained relatively rare.

There were several ways a divorce could go.  The husband could simply declare the marriage over and send his wife away.  No justification was required, but he did have to either return the dowry or pay 18 percent interest on it to his ex-wife’s guardian, which could prove a significant financial burden.  The political and social consequences that might result from angering a wife’s family also made many men think twice.  Women with less influential families might not have that protection.  In a amicable divorce, the soon-to-be-ex-husband would often arrange his wife’s next marriage, so she would spend little to no time unmarried.*

A wife could also divorce her husband, though not so easily.  She had to appear before the archon with a male relative and ask for the divorce.  The one example we have was not successful.  The wife, Hipparete, was seized by her husband, Alcibiades, and dragged back to his house.  Women who appealed to their male relatives to negotiate a divorce generally had more luck.  A wife with an influential family or with a big dowry could hold a fair amount of power in her husband’s house.

There were two other ways a divorce could go.  A husband whose wife committed adultery or was raped was legally required to divorce her.  She then lost the right to participate in religious festivals and usually could not leave her family’s house at all.  The possibility of remarriage was not lost to her,** but it was unlikely.

There was also the case of the epikleros.  A married woman who became an heiress might be required to divorce and marry one of her kinsmen.  A married man who had the opportunity to marry an heiress was quite likely to do so.

A divorce might happen for any of several reasons including an inability to get along, childlessness, possibility of increased inheritance, or adultery, to name a few.  Though it could be as simple as a man sending his wife away, negotiation between families generally played a major part in the proceedings.


*Being unmarried was considered a terrible fate for a citizen woman in Athens.
**See note*

Sources/Further Reading:
Pomeroy, Sarah B.  Goddesses, Whores, Wives, and Slaves. New York: Schocken, 1995.
Powell, Anton. Athens and Sparta: Constructing Greek Political and Social History from 478 BC. 2nd ed. London: Routledge, 2001.
Tetlow, Elizabeth Meier. Women, Crime and Punishment in Ancient Law: Volume 2 Ancient Greece. New York: Continuum International Publishing Group, 2005.
Cohn-Haft, Louis. "Divorce in Classical Athens." The Journal of Hellenic Studies 115 (1995): 1-14.