Length of human pregnancy and contributors to its natural variation. Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ.Fundal height isn’t believed to be as accurate as other measurements, but is still measured during prenatal visits to make sure the fetus is growing as expected. From this point on, the uterus will enlarge about one centimeter per week so that at 25 weeks it measures 25 centimeters, 30 centimeters at 30 weeks, and so on (2). At 20 weeks, it’s likely that the uterus will be right at the belly button (2). Depending on your body type, you may begin to see a “baby bump” as your uterus protrudes out of the pelvis in the following weeks.īy week 16, the top of the uterus will be about halfway between the pubic bone and natural location of the belly button (2). During pregnancy, the uterus stays unseen within your pelvis until about 12 weeks (2). In the absence of any dating technology, measuring the uterus, called fundal height, can give healthcare providers and women and pregnant people a general idea of gestational age (12). Pregnancy providers often break out a measuring tape during prenatal visits and mark down the measurement of the uterus. Your uterus gives clues about your due date Third-trimester ultrasounds are generally used for pregnancy dating only if a pregnant person does not have an ultrasound until the third trimester and the date predicted by the ultrasound is three weeks or more off from the due date estimated by LMP (1,10). Third-trimester ultrasounds are not recommended for pregnancy dating, but in some cases, they may be the only measurement available. Ultrasounds are still considered accurate at predicting birth date if performed before 22 weeks of gestational age (10, 11). When pregnant people have dating scans in and between weeks 11-14, about 70% will birth close to their estimated due date (1). Ultrasounds performed early in pregnancy may be called “dating scans” by your healthcare provider because they look at the size of several different parts of the fetus to determine its approximate gestational age. Research suggests that the most accurate time for an ultrasound is between 11-14 weeks gestational age (9). Ultrasounds, also called sonograms or sonos, can be more accurate than LMP at predicting the date of birth if performed early in pregnancy (9). This is because of two reasons: estimating gestational age is a varied and complicated process and pregnancies naturally vary from person to person and pregnancy-to-pregnancy (1). Ultrasound can more accurately predict the estimated due dateĪbout 4-5% of women and pregnant people birth on their estimated due date (1, 8, 9). Women and people who are breastfeeding/chest feeding can ovulate before their period has returned
Time between fertilization and implantation varies Time from conception to fertilization varies This can cause ovulation to occur earlier or later than day 14 of the cycle The length of the follicular phase of the cycle can vary. Recent hormonal contraceptive use can impact both bleeding and ovulation
CONCEPTION TO DUE DATE FULL
Many factors make this method less than perfect, like cycle length and ovulation date variations (3-7):Ī person may not know their last period start date (tracking your first full flow day with the Clue app makes sure that you do)Įarly pregnancy bleeding can look like a period When an estimated due date is based on your last period, it assumes that you have 28-day cycles and that you ovulated on day 14 (1). A due date based on the last menstrual period doesn’t work for everyoneīecause every body is different, and each cycle is unique, an estimated due date based on the last menstrual period won’t be accurate most of the time, unless you have a scheduled labor induction or cesarean section.
The period is the time when the ovaries are the most inactive as your body prepares for all the hormonal changes of the upcoming cycle (2). It starts with the first day of your period, your LMP.
The first phase of the menstrual cycle is called the follicular phase (3).
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