The Discovery Health web site lists reasons that women don't realize they're pregnant. A lot of them sound like the reasons women undergo late abortions:
10. Fetus is small, inactive, and/or carried toward the back of the womb.
Believe it or not, sometimes babies just don't make a lot of commotion in utero. In addition, some women's babies rest naturally toward the back of the womb, so that motion and kicking aren't easily detected from outside the mother's body. Many women believe that fetal movements are just irregular or strong digestive activity!
9. Recent previous childbirth.
Many women continue to believe that they cannot get pregnant right after giving birth, or while they are breastfeeding. Additionally, menstrual cycles can be irregular, and women can continue to bleed vaginally for several weeks after childbirth. ....
Women who are very active and watch their weight religiously may crank up their fitness routine and/or begin dieting when they notice a weight gain. Because of this, they may not gain weight as their pregnancy progresses.
Stressful jobs, family, and personal lives contribute to irregular periods, and may cause women not to pay attention to their menstrual cycle, or to chalk up missing periods to stress.
6. Fetus is mistaken for a tumor or cyst.
In some cases, particularly when a woman has a history of endometriosis, fibroid cysts, or other tumors of the reproductive system, a fetus can be mistaken for a new or recurring tumor.
Carrying a lot of excess weight can perfectly disguise the "baby bump." Depending on how a woman's body deposits fat, she may have looked "a little bit pregnant" in the past, and neither she nor others notice the extra bulk of a fetus.
4. Inaccurate use of birth control.
Many women are certain they could not be pregnant because they use birth control. ....
3. Breakthrough bleeding.
Some women continue to have period-like bleeding throughout their pregnancy; it may be lighter or irregular, or even coincide with a woman's usual "time of the month." These pseudo-periods can be enough to convince a woman that she's not pregnant.
2. Negative home pregnancy test.
Home pregnancy tests aren't fool-proof. ....
1. History of irregular cycles and/or infertility.
Not being able to get pregnant in the past, a history of missing or irregular periods, and/or being close to menopause can all fool a woman into thinking she couldn't be pregnant. ....
Isn't it unfair that these women don't get the chance to choose abortion? Do we, as a society, owe it to them to give them a chance to back out? If you figure that the typical modern woman finds out she's pregnant about three weeks into the pregnancy, and most states allow on-demand abortion up to about 24 weeks, that means that the typical modern woman gets about 21 weeks to decide if she wants to allow the new human to be born, or to have it put to death via abortion. Ought not women who don't realize they're pregnant until they're very late in pregnancy, or actually giving birth, to have at least a few weeks to decide if they want to parent or not? Why should their sexual activity be "punished with a baby" just because they didn't realize they were pregnant until it was legally past the date to opt out? Ought they not be allowed to request that the newborn be strangled if they don't want to parent it?
(HT: Big Blue Wave