The standard 40-week pregnancy starts counting two weeks before conception
Doctors date pregnancy from the first day of the last menstrual period, so the familiar 40 weeks includes roughly two weeks before fertilisation even happens. Measured from fertilisation, it lasts about 38 weeks. And the word gravid, used for pregnant, comes from the Latin for heavy.
Pregnancy is the period in which one or more offspring develop inside the uterus; carrying twins or more counts as a multiple pregnancy. Conception usually follows intercourse but can also result from assisted reproductive technology. Implantation happens on average 8 to 9 days after fertilisation. For the first seven weeks after that, equivalent to ten weeks of gestational age, the developing offspring is called an embryo, and from then until birth a fetus.
The months are divided into three trimesters of about three months each. In the first, the fertilised egg travels down the fallopian tube, attaches to the uterine wall and begins forming the embryo and placenta, and the chance of miscarriage is highest. Movement can usually be felt around the middle of the second trimester. By 28 weeks, more than 90 percent of babies can survive outside the uterus with high-quality medical care, though they are likely to face serious health complications. Early signs of pregnancy can include missed periods, tender breasts, nausea and frequent urination.
Timing has precise labels. A pregnancy counts as term from 37 weeks; before that it is preterm, and at 42 weeks or beyond, post-term. The American College of Obstetricians and Gynecologists further divides the span into early term from 37 weeks, full term from 39 and late term from 41. Preterm and post-term have largely replaced older words, premature and postmature, which referred to a baby's size and development rather than to the calendar.
Obstetrics also counts pregnancies with Latin-derived terms. Gravidity is how many times a woman has been pregnant, and parity how many pregnancies reached a viable stage; twins count as a single pregnancy and birth. A woman never pregnant is nulligravida, one pregnant for the first time primigravida. During her second pregnancy she would be recorded as gravida 2, para 1, becoming para 2 after delivering. Those who have never carried a pregnancy past 20 weeks are called nulliparous.
Source: Pregnancy
