"When are you due?"
It's one of the first questions people ask pregnant women -- and a fundamental part of ensuring those women receive quality prenatal care.
On Monday, the American College of Obstetricians and Gynecologists (ACOG) joined with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine to release new guidelines for health care providers on estimating accurate due dates for expectant mothers.
"Many obstetric management decisions are based on gestational age, for example, when to induce, or [the] administration of steroids to enhance lung maturity," Dr. James Goldberg, vice chair of ACOG's committee on obstetric practice, wrote in an email to The Huffington Post. "Thus, having accurate and consistent dating is important."
Monday's release marks the first time ACOG has issued a formal statement on how best to ascertain a woman's estimated due date, Goldberg added.
Overall, a high-quality ultrasound in a woman's first trimester is the most accurate method of establishing or confirming the gestational age of the fetus.
Therefore, if information from an ultrasound is available -- or if the date of the woman's last menstrual period is known -- health care providers should estimate the woman's due date and record it in her files as early as possible, using whatever they believe to be the best obstetrical method. (Women are traditionally told their due date is 280 days, or 40 weeks, after the first day of their last period -- a rule of thumb that assumes each woman has a typical 28-day cycle and that each woman ovulates on the 14th day of her cycle.)
If, for example, an ultrasound performed less than 14 weeks into the pregnancy suggests a due date that differs by more than seven days from the estimated due date generated by a woman's last period, the woman's providers should change her due date to reflect her ultrasound. Before the nine-week mark, a discrepancy of more than five days is reason enough to change her due date.
"Some studies suggest up to 40 percent of pregnancies would have dates changed based on [a] first trimester ultrasound," Goldberg wrote to HuffPost.
However, Monday's joint statement from the various medical groups emphasizes that a woman's due date should rarely be changed based on an ultrasound in her second or third trimester -- particularly if she had a previous ultrasound that closely matched the estimate generated according to her last menstrual period.
"I think the main thing patients should understand is that their [estimated due date] is set early in pregnancy and that this does not change based on a third trimester ultrasound," Goldberg wrote. "A common question I get from patients after performing a third trimester [scan] is, 'Are my dates still the same?'"
Monday's statement also outlines procedures for dating pregnancies that result from the use of assisted reproductive technologies, including in vitro fertilization. The estimates determined by a woman's fertility team -- for example, the age of the embryo and date of transfer in the case of IVF -- should be the primary measure used to determine a woman's due date, as opposed to subsequent ultrasounds.
Yet even with improved imaging and dating policies, estimated due dates are just that -- estimates. Only a small number of women -- 5 percent, according to some figures -- deliver on their given due date. Traditionally, babies born as much as three weeks before their due dates, or up to two weeks after, have been considered full term.
In 2013, however, ACOG redefined what it means for a woman to carry her baby to term, in an effort to prevent non-medically necessary inductions before 39 weeks. Babies born between 37 weeks and 38 weeks and six days are now considered "early term." Babies born between 39 weeks and 40 weeks and six days are considered "full term." Babies delivered between 41 weeks and 41 weeks and six days are considered "late term," while those born after 42 weeks or more are considered "postterm."