So called because of her straw mattress, "the woman in the straw" was a term for the newly-delivered mother in early modern England. The woman in the straw was required to lie in this her straw for the entirety of her lying-in period: her "woman's month", or in modern terms, the puerperium. This allowed her to recover from the strain and other more serious medical issues involved in having a child at the time.
The lying-in generally occurred in the same place in which the mother had given birth. This was usually the woman's bedroom; she was in her bed. There were a variety of birthing preparations, put in place for the actual birth and then maintained over the course of the month. For instance, the windows were covered, to keep the new mother in darkness. She was also kept warm and snug: "learned doctors agreed 'that there is nothing worse to childbearing women than than cold air' which risked entering and injuring the womb". In short, the room itself was equipped as a womb, protecting the mother while she recovered.
Staying in the same bed seems to have been a hygienic issue, sort of. The woman lay in the same bed, but also in the same traditional linens on which she had given birth. One would surmise that these linens were full of placenta and a variety of birth-related bodily fluids. Using the same sheets seems to have been done in order to limit the amount of hemmorhage-oriented damage: if the woman stayed in the same soiled bedding, then nothing else was going to be stained. The woman at least experienced the same two-week discharge that women today experience after birth. The Merck Manual describes it like this:
Vaginal discharge is grossly bloody (lochia rubra) for 3 or 4 days, changing over the next 10 to 12 days to pale brown (lochia serosa) and finally to yellowish white (lochia alba).
And that is just the normal reaction after giving birth. As well, at the time there was a much larger risk of hemmorhage and other various complications than we have today, considering that medicine was a much less exact science. Birth was supremely messy that way.
This staying in the same linens was also possibly a religious issue. So, the woman remained in her straw (and her own blood) for her traditional month. During this time, she was expected to remain celibate: probably not a problem for her, considering that she did need actual time for her body to recover, but possibly frustrating to her husband. She could not attend church or receive any of the sacraments--she could not leave the house to witness her own child's baptism!--yet she was supposedly not considered impure. Hmm. I don't know about that. The symbolism implied by bloody linens seems to indicate that she was pretty impure, especially since she was required to go through the rite of Churching* before she could re-enter the community and function normally within it again. Perhaps then these linens were thought appropriate to her state.
So, the new mother did have to recover, both physically and spiritually. Women apparently took a good while to regain their strength after giving birth. They were not allowed to even sit up in bed until two to three weeks had passed, when, "if all went well, the woman would be bathed and changed, and removed from her soiled bedding 'in the straw' ". This was called the "upsitting", or "footing time", and was celebrated as a mark of recovery.
The amount of time spent in the straw varied a bit according to amounts of recovery as well as economic status. It is thought that poorer women were less able to take a full month, since they needed to work in order for their family to survive economically. The housekeeping was traditionally left to the husband during the month; this varied as well.
After her approximate month was up, the woman would be allowed to get up and resume her household duties. She would be churched, and thus take her place in the community. The woman's role had changed over the course of her pregnancy: the birth itself, plus her time in the straw, established her as a mother in addition to being a wife.
* give me a couple days, I'm working on it. Churching is COMPLICATED.
Sources:
http://www.merck.com/pubs/mmanual/section18/chapter254/254a.htm
Cressy, David. Birth, Marriage, and Death: Ritual, Religion, and the Life-Cycle in Early Modern England.New York: Oxford University Press, 1999. Quotes from pp. 83, 86.