Diagnosis of pregnancy and testing
Pregnancy is counted as 40 weeks, starting from the first day of the mother’s last menstrual period. Your estimated date to birth is only to give you a guide.
The unborn baby spends around 38 weeks in the womb, but the average length of pregnancy (gestation) is counted as 40 weeks. This is because pregnancy is counted from the first day of the woman’s last period, not the date of conception, which generally occurs two weeks later.
The moment of conception is when the woman’s ovum (egg) is fertilised by the man’s sperm.
All urine or blood pregnancy tests rely on the detection of human chorionic gonadotropin (hCG) produced by the placenta. Levels increase shortly after implantation, approximately double every 48 hours (this rise can range from 30% to 100% in normal pregnancies), reach a peak at 50–75 days, and fall to lower levels in the second and third trimesters. Pregnancy tests are performed on serum or urine and are accurate at the time of the missed period or shortly after it.
Compared with intrauterine pregnancies, ectopic pregnancies may show lower levels of hCG that plateau or fall in serial determinations. Quantitative assays of hCG repeated at 48-hour intervals are used in the diagnosis of ectopic pregnancy as well as in cases of molar pregnancy, threatened abortion, and missed abortion. Comparison of hCG levels between laboratories may be misleading in a given patient because different international standards may produce results that vary by as much as twofold.
hCG levels can also be problematic because they require a series of measurements. Progesterone levels, however, remain relatively stable in the first trimester. A single measurement of progesterone is the best indicator of whether a pregnancy is viable, although there is a broad indeterminate zone. A value less than 5 ng/mL (16 nmol/L) predicts pregnancy failure while a value greater than 25 ng/mL (80 nmoL/L) indicates a pregnancy will be successful. There is uncertainty when the value is between these two points. Combining several serum biomarkers (beta hCG and progesterone) may provide a better prediction of pregnancy viability.
Pregnancy of unknown location is a term used to describe a situation where a woman has a positive pregnancy test, but the location and viability of the pregnancy is not known because it is not seen on transvaginal ultrasound.
Manifestations of Pregnancy
The following symptoms and signs are usually due to pregnancy, but none are diagnostic. A record of the time and frequency of coitus is helpful for diagnosing and dating a pregnancy.
Amenorrhea, nausea and vomiting, breast tenderness and tingling, urinary frequency and urgency, “quickening” (perception of first movement noted at about the 18th week), weight gain.
B. Signs (in Weeks From Last Menstrual Period)
Breast changes (enlargement, vascular engorgement, colostrum) start to occur very early in pregnancy and continue until the postpartum period. Cyanosis of the vagina and cervical portio and softening of the cervix occur in about the 7th week. Softening of the cervicouterine junction takes place in the 8th week, and generalized enlargement and diffuse softening of the corpus occurs after the 8th week. When a woman’s abdomen will start to enlarge depends on her body habitus but typically starts in the 16th week.
The uterine fundus is palpable above the pubic symphysis by 12–15 weeks from the last menstrual period and reaches the umbilicus by 20–22 weeks. Fetal heart tones can be heard by Doppler at 10–12 weeks of gestation.
The nonpregnant uterus enlarged by myomas can be confused with the gravid uterus, but it is usually very firm and irregular. An ovarian tumor may be found midline, displacing the nonpregnant uterus to the side or posteriorly.
Ultrasonography and a pregnancy test will provide accurate diagnosis in these circumstances.