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One such relatively recent study tells us that implantation does not always take place on day 7 after ovulation. In fact, it very rarely does. This study by AJ Wilcox accurately determined the day of implantation by very sensitive pregnancy test (HCG) measurement compared to ovulation.

HCG hormone starts being produced when the pregnancy implants into the uterine lining. The findings in normal women trying to conceive include:

The first appearance of HCG (implantation) occurred 6-12 days after ovulation
84% of the pregnancies implanted on days 8-10 after ovulation
early pregnancy loss increased with later implantation –
implantation early pregnancy loss rate
13% by day 9
26% on day 10
52% on day 11
86% on day 12 or more
Overall the total pregnancy loss up to 6 weeks was 25%. Now that seems very high to most people but keep in mind many of these pregnancy losses occurred so early that women often were not aware they were even pregnant. The normal early pregnancy loss rate that most women know about is 15-18% of clinically recognized pregnancies so almost 40% of all pregnancy loss is unrecognized.

When is the best time to have intercourse in order to get pregnant?
Wilcox had also performed previous work in 1995 (2) which demonstrated that pregnancy only occurs if intercourse occurs within the 6 days prior to and including ovulation. Intercourse after the day of ovulation does not result in pregnancy. The probability of conception ranged from 10% when intercourse occurred five days before ovulation to 33% when it occurred on the day of ovulation itself.

This is the data that suggests the best timing of intercourse in order to conceive is day 10, 12, 14, and 16 (in case of late ovulation) of a 28-day cycle or days -4, -2, 0, +2 in relation to expected ovulation in the case of cycles different than 28 days.

When should I begin testing with home urine tests?
Most home pregnancy tests that check urine beta-HCG are sensitive to 20-25 mIU/ml of HCG. The general rule of thumb is to test at the time when you are a day late for your menses or about 15 days after ovulation. The test can be positive anywhere from about 2-3 days prior to a missed menses to 4-5 days after.

Testing really does depend upon how regular your menses are or in other words how regular ovulation occurs. If you tend to be late (longer than 28 days) with your periods or the timing of menses varies by several days each cycle, then it is better not to waste pregnancy tests by testing at days 28-29 after the last period starts.

Are ovulation prediction tests worth doing?
Pelvic ultrasound looking at the ovaries is considered the gold standard in ovulation prediction in a research setting but obviously in a practical application of a woman trying to conceive, ultrasound is not used. Urinary LH testing has been shown to have a 100% correlation with ultrasound as far as predicting the timing of ovulation (3). In that same study, they looked at cervical mucous changes, basal body temperature (BBT) charts and salivary ferning. Cervical mucous changes only had a correlation of 48%, salivary ferning correlated 37% and BBT has a 30% correlation.

For the home tests that measure LH surge that immediately precedes ovulation, detection of LH occurs at above 30 mIU/ml. This means that women with polycystic ovarian syndrome who have slightly higher resting LH values are still not falsely detected as ovulating. The urinary LH spike occurs about 24-36 hours prior to ovulation so it is very useful for women trying to conceive because it gives them a window of warning. The test will show positive for one or two days and rarely 3 days if you catch a spike right at the beginning and it is a large release of LH.


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