Conception dating

22 Mar

The expected date of delivery (EDD) is one of the earliest pieces of information a pregnant woman requests once pregnancy is confirmed.

In order to calculate the EDD, the practitioner must know the median length of normal pregnancy and the last menstrual period (LMP) or ultrasonographic estimation of gestational age (GA).

Size can be misleading in the presence of multiple pregnancy, uterine fibroids, or a full bladder.

Tape measurement of the symphysis-fundus height may be useful up to 28-30 weeks' gestation, beyond which it becomes too inaccurate for dating.

The estimation of pregnancy dates is important for the mother, who wants to know when to expect the birth of her baby, and for her health care providers, so they may choose the times at which to perform various screening tests and assessments,such as serum screening, assessment of maturity, and induction of labor for postdate pregnancies.

The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography.

The first 2 are subject to considerable error and should only be used when ultrasonography facilities are not available.

To further complicate matters, 10-45% of pregnant women cannot provide useful information about their LMP, and 18% of women with certain menstrual dates have significant differences between menstrual and ultrasonographic dating.The date of the first documented positive pregnancy test and the beta-human chorionic gonadotropin (b HCG) level may help ascertain the minimum gestational age.In women who conceived following assisted reproduction techniques, the date of embryo transfer is known and may date the pregnancy accurately.The accuracy of menstrual history in women with a history of oligo-ovulation, such as those with polycystic ovarian syndrome, should be questioned.If conception occurred while oral contraceptives or long-acting progestogens were being taken, the LMP cannot be used because it has no relation to the time of ovulation.