When pregnancy begins depends on how “pregnancy’ is defined.
Background
Does it begin when the sperm first penetrates the egg? Does it begin when the male and female pronuclei (see picture below) containing the chromosomal material move close together and decondense in the newly fertilized egg?
2 circular structures (pronuclei) in the center contain maternal and paternal DNA
Sperm and egg were put together 16 hours ago
- Does pregnancy begin when the male and female chromosomes pair up with each other and the fertilized egg subsequently divides into a 2-cell embryo?
- Does it begin when the embryo implantation begins about 6 to 8 days after ovulation?
- Or, does it begin when the woman recognizes that she is pregnant after her missed menstrual period and positive pregnancy test?
HCG levels in pregnancy
HCG, or human chorionic gonadotropin, is a hormone made by the pregnancy that can be detected in the mother’s blood or urine even before the woman’s missed period. This hormone is what we look for with a “pregnancy test”.
HCG is first detectable in the blood as early as 7-8 days after ovulation by very sensitive HCG assays (research assays). In real life, blood pregnancy tests will be positive (> 2 mIU/ml) by 10-11 days after HCG injection or LH surge.
In general, the HCG level will double every 2-3 days in early pregnancy.
85% of normal pregnancies will have the HCG level double every 72 hours.
HCG levels peak at about 8-10 weeks of pregnancy and then decline, remaining at lower levels for the rest of the pregnancy.
There is a large variation in a “normal” HCG level for any given time in pregnancy.
Pregnancies destined to miscarry and ectopic (tubal) pregnancies tend to show lower levels (eventually), but often have normal levels initially.
Learn about pregnancy after ectopic pregnancy
Some normal pregnancies will have quite low levels of HCG – and deliver perfect babies. Caution must be used in making too much of HCG “numbers”. Ultrasound findings after 5-6 weeks of pregnancy are much more predictive of pregnancy outcome than are HCG levels.
For women that have had an HCG injection as part of their fertility treatment, the hormone takes about 5-14 days to clear from her system, depending on the dose and the individual woman. This can causes problems with interpretation of pregnancy tests done earlier than 14 days after an HCG injection.
HCG levels from normal singleton pregnancies (and twins below)
Levels are listed for various days after the ovulatory HCG injection or LH surge
First (same as Third) International Reference Preparation was used
“High” is highest seen in this group of pregnancies
“Low” is lowest seen in this group of pregnancies
“#” is the number of tests done for that day in this group of pregnancies
The values below are from a group of 53 normal singleton pregnancies. Because this is a very small group of patients and because lab assays vary, it is important not to rely on these values to determine whether your pregnancy is viable, a single, a twin, etc.
- Check with your physician about your own levels
| Day after HCG or LH | Average mIU/ml |
High mIU/ml |
Low mIU/ml |
# |
| 14 | 48 | 119 | 17 | 12 |
| 15 | 59 | 147 | 17 | 18 |
| 16 | 95 | 223 | 33 | 23 |
| 17 | 132 | 429 | 17 | 21 |
| 18 | 292 | 758 | 70 | 19 |
| 19 | 303 | 514 | 111 | 23 |
| 20 | 522 | 1690 | 135 | 13 |
| 21 | 1061 | 4130 | 324 | 12 |
| 22 | 1287 | 3279 | 185 | 22 |
| 23 | 2034 | 4660 | 506 | 13 |
| 24 | 2637 | 10000 | 540 | 16 |
The information in the table above is part of a study carried out by Dr. Sherbahn that compared HCG levels from single, twin and heterotopic (combined intrauterine and ectopic) pregnancies.
Ultrasound pictures in early pregnancy:
Transvaginal ultrasounds of normal pregnancies at 5.5 weeks of gestation
Gestational sac (black area) is seen with yolk sac inside. An identical (monozygotic) twin pregnancy at the same stage is shown as well as an early twin pregnancy showing non-identical (fraternal) twins.
