The length of a human pregnancy can vary naturally by as much as five weeks, according to research published online August 7 in the journal Human Reproduction. Normally, women are given a date for the likely delivery of their baby that is calculated as 280 days after the onset of their last menstrual period.

We humans are able to crank up our metabolism to about twice its normal level and sustain that turbo mode for quite a while. In fact, pregnant women’s metabolism runs at twice the normal level by about the sixth month. By nine months, as the fetus’s energy needs increase, the rate is pushing close to 2.1 times normal.

Pregnancy lasts for about 280 days or 40 weeks. A preterm or premature baby is delivered before 37 weeks of your pregnancy.

Humans are born 12 months too early. Gestation should be 21 months. Humans evolved to become the pre-eminent animal in the world, but our big brain, bipedalism, and small female pelvic outlet have caused us to pay the price of being born too soon with all of its disadvantages.

Pain during labor is different for every woman. It varies widely from woman to woman and even from pregnancy to pregnancy. Women experience labor pain differently — for some, it resembles menstrual cramps; for others, severe pressure; and for others, extremely strong waves that feel like diarrheal cramps.

The full list, in no particular order, is as follows:

• Shingles.
• Cluster headaches.
• Frozen shoulder.
• Broken bones.
• Complex regional pain syndrome (CRPS)
• Heart attack.
• Slipped disc.
• Sickle cell disease.

While slightly more than half said having contractions was the most painful aspect of delivery, about one in five noted pushing or post-delivery was most painful. Moms 18 to 39 were more likely to say post-delivery pain was the most painful aspect than those 40 and older.

Mammals, including human beings, have the ability to give birth while completely unconscious – even during sleep. This is due to something called the fetal ejection reflex (FER). The FER occurs when a woman’s body spontaneously, and with no effort, pushes out a baby quickly and efficiently.

The labia and perineum (the area between the vagina and the rectum) eventually reach a point of maximum stretching. At this point, the skin may feel like it’s burning. Some childbirth educators call this the ring of fire because of the burning sensation felt as the mother’s tissues stretch around the baby’s head.

With an epidural, you might be able to feel contractions — they just won’t hurt — and you’ll be able to push effectively.

During childbirth, pressure from the baby’s head can fracture the coccyx, or tailbone. A fractured coccyx can be quite painful and symptoms can take months to subside.

Your pelvic bones will slightly separate from each other, and it’s this separation that allows a baby to move through your pelvic joints. But although the ligaments around your pelvis will relax in preparation for childbirth, your pelvis shape doesn’t change.

Your body secretes hormones during pregnancy that soften the pelvic ligaments, allowing the pelvis to open a bit so your baby can pass through during labor and delivery. The pelvic floor muscles spread apart as uterine contractions move your baby down the birth canal.