How accurate is your Due Date?
How are due dates calculated?
The standard calculation is called Nägele’s Rule. This
was developed in the 1850’s by a Dr. Nägele, who
determined that the average human pregnancy was 266 days from conception, or
280 days (40 weeks) from the start of the last menstrual period. To calculate
this, begin with the first day of your last period (LMP). Add 7 days, then subtract 3 months.
((LMP + 7
days) - 3 months) = Expected Date of Delivery
EX: ((April 1 + 7 days) - 3 months) = January 8
This “rule” doesn’t take into account
the fact that many women are uncertain of the date of their last menstrual
period, not all women have 28 day cycles, and not all women ovulate on day 14
of their cycle.
Another Calculation
In 1990, researchers (Mittendorf, et al) re-examined this issue for modern
American women. Results indicated that, for first-time Caucasian moms, an
average pregnancy lasts 274 days from conception (approximately 288 days from
the last menstrual period).
This research and other studies also
show us many other factors influence the length of pregnancy, including:
mother’s age, weight, ethnicity, prenatal care, prenatal nutrition and smoking,
number of prior pregnancies, and more.
Mittendorf shows an average pregnancy is 269
days for mothers who’ve given birth before. Non-Caucasian moms have shorter
pregnancies than Caucasian moms; for example, African-American women average
266 days.
So, to calculate “Mittendorf’s Rule”. Begin with the starting date of the last menstrual
period. Add 15 days for first time Caucasian mom, or add 10 days if you’re
non-white or this is not your first baby. Then subtract 3 months.
((LMP + 15
days) - 3 months) = Expected Date of Delivery
EX: ((April
1 + 15 days) - 3 months) = January 16
What other methods can tell what the
gestational age is / how many weeks pregnant someone is?
Those calculations predict a due date
based on the date of conception. If the date of conception and the date of the
last menstrual period are unknown, there are several other ways to determine
how far along the pregnancy is.
ü
Ultrasound: Ultrasound
can be used to examine the baby’s development, and determine his/her age based
on which systems are fully developed. If ultrasound is performed in the first
trimester, it can indicate fetal age within a range of 3 – 5 days. Later in
pregnancy, it is less accurate. Up to 20 weeks, the margin of error is 7 – 10
days. By the third trimester, an ultrasound date can be off by as much as 3
weeks in either direction. Therefore, a baby that appears “term” (40 weeks) may
be anywhere from 37 – 43 weeks.
ü
Heart Tones: Fetal
heartbeat can be heard through Doppler starting at 9-12 weeks and by
stethoscope at 18-20 weeks.
ü
Fundal height: A physician or midwife can
measure mom’s belly, specifically the distance from the symphisis
pubis joint to her fundus (the top of her uterus). Fundal height can indicate the size of the baby, which can
give insight about fetal age. Typically, from week 24 to week 34, fundal height in centimeters correlates with weeks of
gestation. For example, at 28 weeks, the fundus is
probably about 28 cm. This is not an exact measurement, and several factors can
affect its accuracy.
ü
Quickening: Some believe
the baby will come five months after quickening, the first time the mother
feels the baby move. This is hard to evaluate, as women can be more or less
sensitive to these sensations, and may notice them at different times in their
pregnancies. (First time mothers typically notice movement around 18-20 weeks.
Mothers who have been pregnant before notice it as early as 16 weeks.)
“Expected Date of Delivery” versus Actual Date of Delivery
Even
if you know your exact date of conception, and the exact gestational age of
your baby, and know the “average” length of pregnancy, it is still difficult to
predict exactly when your baby will
be born. A normal pregnancy can last anywhere from 37 to 42 weeks. Only 4% of
babies are actually born on their “due dates”. 6-10% of babies are born
early – prior to 37 weeks; 4-14% of pregnancies last more then 42 weeks.
So,
when will your baby be born?
If
this leaves you very confused, just realize that predicting due dates is a very
inexact science.
I
always tell my students: make sure you have everything ready for your baby and
for your birth by two weeks before your due date. But then have some really fun
and exciting projects to keep you busy for up to two weeks after your due date,
that way you don’t feel too frustrated if your baby comes a little later than
“expected.”
For
more on due dates, read: Calculating
Due Dates and the Impact of Mistaken Estimates of Gestational Age
By Janelle Durham, 2002. Sources: “How long is too long?” by
Penny Simkin, Childbirth Forum, Spring 1993. Abstract for “The Length of Uncomplicated Human
Gestation” by Mittendorf et al Obstetrics
& Gynecology, V.75, N.6, June 1990. “When will my baby be born?” found
at http://pregnancy.about.com/library/weekly/aa042197.htm
“Pregnancy past your due date” by Terri Isidro-Cloudas
on americanbaby.com
c.
2004, Janelle Durham, http://www.transitiontoparenthood.com/